I think of Tumblr as being the JRPG of Facebook. Users of both sites love to statwhore, but on Facebook you can count on a greater percentage of your friends being people you actually know. Since Tumblr has a more specific focus on content, views, and reblogs, it's less about the people and more about the numbers, and when you don't know the people that are reblogging your content, those people are really nothing more than numbers.
I have a friend that had something go half viral on Tumblr - an image that gave her a boost from several dozen followers to several hundred. When all you're seeing is an image, and you follow someone based on that, you don't really get an idea of who you're following, other than knowing what their general interests are. A few posts can go viral on Facebook, but it's not nearly as common.
To do another quick comparison, Twitter has much more focus toward text, which makes it more of a shorthand blogging site. While you can hypothetically do the same on Tumblr, it's just not what makes up the majority of the content. Usually when I find a lengthy article on Tumblr, it's some preachy bullshit from some self-righteous internet activist that may have been reblogged but is seldom talked about. Perhaps it has to do with people reblogging in an attempt to "collect" content that they like rather than engaging with the content creator.
As for Newgrounds, we have a lot of stuff here, but the userbase is too small to be considered a service the way YouTube or Tumblr is, and that makes it a community almost by default. There are small circles of programmers, animators, artists, and musicians here that know each other, and the circle is small enough that you can find your way into it if you want to.
I'm curious what Instagram is like, since it sounds very similar to Tumblr. I've never been a frequent visitor of either, but I'd like to hear someone compare the two.