I have both and they sort of got big whilst I was going through school.
I think I had a Myspace at 14, Facebook at 15 and Twitter at 16.
Although they've never done me any harm, I do think that they have had a detrimental effect on young people.
School children can be cruel as it is and Facebook and Twitter act as further outlets for bullying; away from the playground.
I wish they could be tighter on the age restrictions as well. My 9 year old cousin has a Facebook and I refuse to accept him. Facebooks full of stuff that young kids shouldn't be looking at and I think it's half the reason for the over sexualisation of young kids these days. Constantly taking "sexy" profile pictures and trying to out-do their 'friends'.
That plus it's a mega distraction from revision and even going out and having a life.
I do love it for socialising and keeping in touch with old friends though. I'm very conflicted on how I feel about Social Networking tbf.