Recently Facebook overtook google.com as the most visited site on the 'net in the USA.
There have been murmurings for quite a while of the implications of Facebook in regards to SEO, it is a whole different ballgame. We like things like straight up honest links. We don't like redirects of any kind.
And sometimes the since people on Facebook can be real people that you actually know from the real world, you don't want to contaminate that world with anchors and key words, and it should be more about cool stuff that you might find out there.
Of course, saying that won't stop the marketers trying to getting in there, and eventually it will just become a commercial hell, and we'll have to optimise for it as well, or somebody else.