I use the Like button as a bookmark. Basically if I see a page that doesn't have a like on it from me, I know I haven't read it yet. I don't always go to a series as soon as it updates, sometimes 3 or more updates have posted before I go back to it. So the Like shows me where I left off. However, I also know that a like on a page can help boost a series some, so knowing that I like everything I read for the most part.
Receiving a like is nice however like it's been said before, it can mean anything. So yes, it does make me happy to see an engagement but it's not as meaningful to me as a sub. When I see a sub, I think that something caught their attention enough to want to be notified for more. And comments means more to me than a like as well. An actual dialogue I can get more out of. A like, I don't want to take it for granted, but it's mostly a stat, a number, a piece of data.
Shares are really important, because it help spread our comics through a genuine method, word of mouth. A reader likes it enough to want to share it on their social media. That's awesome! However, the Tapastic dashboard is a bit broken for being able to show us creators how many shares we actually get.
I personally share a lot of the series I follow. A big chunk of my Twitter tweeks are Tapastic episode update shares. I do notice that my followers like and retweet a couple of them and no one has mentioned it being annoying yet so I keep it up. Also I noticed just toady, the Tapastic staff finally changed the tag at the end "via @tapastic" to "via @tapas_app" XD lol I've been changing it to the later tag by hand for nearly a year now. lol
I do agree that perhaps a little mention needs to be given to readers to remind them to share, however it shouldn't be in their face loud about it. It's still up to the creator to promote themselves and shouldn't rely on readers to share for them, just an extra cherry on top deal. Perhaps a line in the episode description mentioning "Help support us by sharing" or in the series description? I don't think having the "Like/Comment/Share/Sub" at the end of the episode or in the series banner or really in the custom banner helps. I personally think it make the series look somewhat desperate for those engagements. And as a creator that did do that in the beginning, it really didn't help all that much.
There are other better method that I have found to growing my series rather than asking for engagements. The collab for example, gives a good bump each year. MORE PEOPLE SHOULD TRY TO SET THEM UP! HINT HINT Also livestreaming with multiple creators and organizing art request streams together. Creative stuff like that where readers can have fun with you has been better than asking for engagements.
Like everything else in webcomics, with time and growth these engagements naturally happen on their own.