The power of 'I like this' in social applications
One of the things that turns plain applications into social applications is the ability to get feedback from other people, the ability to know that other people are appreciating your stuff. I think it follows as a corollary that if you want to build a popular social application, getting the feedback right matters a lot; the more feedback people get, the more likely they are to continue using your social application.
To be specific for a bit, let's talk about Flickr (partly because I have experience with it as a user). The leading feedback mechanism on Flickr is leaving comments on other people's photographs. This has the drawback that writing a comment requires you to either have something to say about the picture or be willing to leave a meaningless noise comment; otherwise, you won't do anything, reducing the level of feedback and thus the 'stickyness' of Flickr as a whole.
(You can in theory mark a photograph as a 'favorite', but this is a bad feedback mechanism for various reasons, including that people like a lot more pictures than are their favorites.)
Experiencing this effect directly has led me to a thesis: in a social application, it's useful to give people a lightweight, socially acceptable way of saying 'I liked this', 'thumbs up', or the equivalent, without forcing them to find something to actually say (or to clutter up actual comments with 'me too'). People are not necessarily articulate about things, they don't necessarily want to write, and forcing them to do so in order to create feedback lessens the amount of feedback that they leave. Reducing the amount of work and effort that it takes to create feedback means that you'll get more of it, with the accompanying good effects.
(In support of this thesis, I note that a number of recent social applications have an explicit 'I like this' feedback option, such as Facebook and Tumblr. Facebook even illustrates how you can condense such feedback so that it doesn't take up as much space as comments.)