Helping people stay in touch socially is important to humans. The engagement figures alone tell me that it's important (to them).

My parents and grandparents see more pics of my kids and feel more connected to their grandkids' random life experiences. That's important (to me and to them).

Jzw's point on narrowing your focus is worth mentioning here: http://www.jwz.org/doc/groupware.html Facebook isn't helping me there, but it's helping others, and that's important to them.