[jmgrosen]

I think Instagram (along with Twitter) is used a lot to interact with a wider base of people than just your friends, supporting your idea of image-based networks, but I don't think it has to do with their particular media. Reddit, and other forum-like sites, are popular in certain cliques (mainly tech-savvy ones -- anime/manga fans, gamers, etc.), but hasn't really broken out in mainstream teen culture. My hypothesis is that teenagers like the more personal, if you will, aspect of Twitter/Instagram/Facebook/etc. in that they are very user-based -- even if you find content based on a hashtag (say, on Twitter), who posted it is just as important as what they posted, unlike forums, where content matters significantly more than its poster. (How many Reddit usernames do remember? Now what about Twitter?)

I think pseudonymity is perceived in a very different manner than anonymity. With a pseudonym, you can piece together the scraps of information someone leaves and maybe make a good guess at who that account is; at least, that's how one feels when using a consistent pseudonym, even if it's unrealistic. With anonymity, you have no sense of responsibility for your actions, because each one post is unlikely to reveal much on its own.

Again, I probably have a skewed view of things, but hopefully this was at least thought-provoking!