Spamming is definitely not worth it, because other users may flag your post and then I get to remove it. 
In other words, I selfishly ask for less spamming.
As someone who sees those spamming users, nothing drives me away harder than seeing the same person spamming a link over and over and over and over, clogging the forums in a hard attempt to get views.. I get putting yourself out there, but it'd really be more beneficial to just make a thread with an interesting title to grab attention
Tbh I often think about asking the mods to put a limit on how many times you can share a link within a day each time I see this
I mean you can see how many clicks links get here on the forums to see what types of promotion actually work and which are just link graveyards. No one wants to feel like their interaction is just a stepping stone.
Nah, not really lol. The megathread was created to combat this some, but this comment from the forum mod partway down nails the heart of the issue:
"I suspect the threads are so successful with users because it feels like you're doing something, while really you're just wasting your time."
Basically it's a good way to feel like you're putting yourself out there and promoting your work... but you're doing so in a really inefficient place and way lol. Marketing poorly and often to the wrong audience.
Like while it's not a bad idea to include links to your series where appropriate (such as in a "do a thing and share your comic" type thread, or perhaps to use a specific episode as an example in a discussion and linking to said episode), it's not... suuuuuper effective, and especially the threads that are in all honesty just giant link graveyards really rarely get any clicks. You can see how many times a link is clicked on these forums, usually it's like maybe 1 or 2 in those threads, even after days or weeks.
What is really useful though is making sure to include a link to your profile or series in your Forum Account's "about me" section. If you become a regular here, it's not uncommon for people to check out your profile and see what you do, and making it easy for people to find that from anywhere you post is 
Of all the times I've shared my comic in those link threads... I've had maybe two link clicks total! (I should probably just stop trying haha) So no... I don't think it's really worth it. But engaging with viewers, talking about your story, sharing your art in appropriate threads, and building connections with other users... that can absolutely get you more users! It's not bad to share your link on certain occasions, but do it genuinely and appropriately! I find that having it in your forum bio probably works better!
What drives me crazy about promotion theads is: why does nobody stay and check others? I remember when I started here, I always checked others because it was so OBVIOUS to me that for those threads to work EVERYONE should stay and check one or two comics... It's really basic logic, either everyone checks or nobody does. I felt stupid checking so many until I realized nobody cared to support others. Still, funnily enough, some of the friends and their really great comics I've got here, I've met them there. But the key is interaction, again, not mindlessly promoting.
I only post my link when applicable. If it's a comic thread then i obviously won't post my novel link. I check the amount of replies on promo posts and check the date as well. I usually post mine if others have recently and i usually get some clicks. I prefer the more engaging posts where we actually talk about our stories and i make threads that encourage people to talk to each other as well as encourage others. I don't see anything wrong with casting your net wide under a lot of posts, but you have to be smart about where you drop your story.
There is a question as to how effective self-promotion threads are. It can be difficult to get clicks, but I can say I discovered some hidden gems I'm not sure I would've found otherwise.
As to whether or not it's "polite". Well. I have seen people silently link their comics seemingly without even reading the thread (i.e. "link your own comic and rec another" and they only plug their own).
Other than that, I'd be a hypocrite for saying self-promoting bugs me, hah. People are just excited to share their work.
I sometimes forget to go back and look lol. But i usually see the same people in the same thread so somewhere in there i get to reading their story. Also, i think it's more beneficial in the long run to genuinely sub to someone's story if you're actually going to read it. I appreciate the help but i'd prefer actual readers or even people who'll like my chapters when i update. It makes no sense to come here to get like 50 subs but only have not even a quarter of the likes, you know?
My rule is once per thread. If a promotion thread is new, I may add again. BUT my favorite ones are where you "read for read" or "sub for sub" because I've found new friends AND new comics that way. I mean, it's not impolite to just post the link, but it's really fun when I can privately message someone, or even just comment on their work and gush about what they did well!
I'm going to speak to novels for now:
Daily, no. But if the topic applies to your novel and you have something more than just a link to give, I think it's okay. I've actually gained readers from posting my link here. I've never promoted elsewhere and the forums have helped me there.
But posting over and over again in every topic that you think pertains to your work can get tiring ALONG WITH the same topics. It tends to dig up old topics that were ready to rest.
Novels seem to be a doomed category for repetitive topics like "Post your novel here" "I'm new, here's my novel, post yours" or "Post your novels under 200 subs".
It's nice when there's actually an in-depth topic to work with for novelists.
I think the same about subs, having a caring fan base that actually reads and comments is the best thing about this web.
By the way, I want to reiterate what @Rhonder said about the link in the bio. I usually check them constantly! Actually I feel I'm missing a lot of comics of this thread.
This one is kind of an interesting subtype tbh. I do think it's appropriate most of the time to make a forum post announcing a new project like that, but you're right that often times they're like "oh but you can also put yours too!" which... I dunno, seems unnecessary in some ways? If you're gonna direct attention to your own project give it room to shine imo! There are numerous other link dump threads without turning one's own soap box into one 
I need to go add mine and i never would have thought to do that.
I like threads like this because these forums aren't always about post dumps, but actually talking with other people and helping each other out. I actually haven't posted my link much today, I've spent it mostly talking to people
I think this is why a lot of people see them as spam..! Because the threads have become drop and go instead of people hanging around to chat and ask questions about each other's content. In and of itself it's not necessarily spam, but it can definitely feel like it when you see it over and over!
Engaging with others is always a lot better than spamming a link I think. I really like mutuals because it creates a better dynamic for engaging vs. following a bunch of people I don't feel a connection with. I've made so many friends in the comic community this way, and found a lot of great comics I'd never have looked at otherwise just because they wouldn't have appealed to me on a description only basis. I highly encourage others to engage vs. just posting a link and running as well since it's through engagement that connections and friendships are made 