Isn't it because the "pagerank" of your page increases by how many people "like" your page instead of how many other sites link to your page. This really is quite interesting because it's easier to press Like than it is to write an intelligent post with a lot of words that links to the site that you like. The downside seems that every site has to buy in and put the like button on their site, but it looks like it's already too late and there's not much choice.