I may only mention something that's similar or related to my comic (without providing a link to mine), only if the other creator is a good friend of mine and we have already read each other's works (and even then, I try not to go on that long about it). If it's someone I don't really know, but there's some kind of funny similarity I may want to point out, I would PM them so it wouldn't seem like a public promotion (and for such a thing, I wouldn't be doing it to have someone read my comic in return. I've once commented in a PM to someone how eerily similar our comic themes seemed, and we had a good laugh about it).
I really find that the best place to promote is twitter though. If other people like your promotions enough, they'll respond with likes, follows, and most importantly, retweets (which really helps if they have a lot of followers). It's separate enough from tapastic, that you don't have to feel as weird or invasive about just mentioning your comic, but even still on a platform like that, you have to avoid being spammy. I just think you can be a little more open, especially when it comes to comic chats like #ComicBookHour and #WebComicChat.