I haven't exactly understood it yet, but it sounds really interesting. Most of the time, when I specify on one site that I like something, I really want other relevant sites to automatically get this information and maybe to offer me something similar.
Furthermore, Google Buzz, or MySpace, or Facebook, or Netflix can to a certain degree calculate the likelihood for my friends to like the same thing I like on their sites and adjust their offer (context adds, etc.) for my friends. This is what a "smart web" environment is.
Google currently collects one's preference behinds the scene through data mining and displays adds accordingly. I don't mind, I rather like it: it help finding relevant info and products.
What Facebook is going to do is different, because a) I express my preferences manually by clicking on "Like" button; b) Facebook is going to team with other big portals, have them display Facebook "Like" buttons and share information. I think it would be good for Google to follow the trend, to introduce "GoogleLike" buttons (may be even placed on Google Toolbar) and to share this info with other portals and social sites, including Facebook. It seems to be a win for everybody.
What do you think?