But with the Open Graph protocol you do get the ability to tie disparate pieces of data together. For instance, let's say that I examine someone's tweet stream for links. If I fetch the OG data for each of those I can tell you whether they tweet most often about people, or movies, or whatever. I can show you a thumbnail display of images representing the things they linked to.

Open Graph is most definitely semantic web. Liking is just one possible application of that semantic data.