I've wondered about this as well. When I was going through my 20's there came big splits in my friends pool in terms of common interests, first the division of friends who were married and those who weren't, and then the much bigger split of people with kids and those without. I noticed that my focus changed at those times as did others and the folks who weren't married or didn't have kids were really not interested in the stuff I was focused on.
We now have a generation of people going through this with Facebook and it doesn't surprise me if suddenly one group of friends is not as relevant as they used to be, and that a system like Facebook can't easily pick that up.