One of the reasons I am not on FaceBook is that the mental acuity of the user mob is not, on the average, very high. Obviously there are intellectual standouts. Just as obviously, it is easier to stand out when you are surrounded by dolts.
Another reason I'm not on FB is because of the security risk it represents. When I was still employed we had to be aware of all of the nefarious behavior facilitated by FB. So I avoid it in general. And to be perfectly honest, have little or no curiosity.
Yet another reason is that it is a general social network but I'm a curmudgeon and therefore an anti-social kind of guy. So I don't care to participate in FB. It is contrary to my world view.