First, this gives facebook a huge database of the Internet. Essentially, if they wanted to release a stumbleupon equivalent, they could. Or if they wanted to target ads even closer to your interests, they could. This makes me feel (more) nervous about facebook.

Second, I've always been really interested in finding the best webpages on the Internet. Unfortunately, this is another one of those metrics that conflates popularity with quality. If we assume that X% will press the like button (where X is different for every website), and Y people visit the site, then we have a total number of likes = X*Y/100. But since X only goes from 1-100, it's pretty easy to see that popularity has a much larger affect on how many likes a site will get than how good it actually is.

Essentially, the number of likes is a meaningless statistic (except to measure popularity).