As someone who had facebook for awhile & deactivated my account, here are the reasons why:
1) to avoid nisayaon/temptation of accepting opposite gender “friend” requests.
2) a friend of mines account was hacked and posted things by hacker that she never would have posted.
3) to avoid seeing photos and language that are not with my hashkafa
4) ayin hara -its easy to get caught up with the whole fad of posting pics of your latest vacation or family pics and it really goes against the jewish concept of not flaunting everything you do…