Personally, I came off FB because I felt it was interfering with my desire to create a great social life off-line. And that's not the case with everyone of course. For me it has more to do with me being relatively new to my area. I wanted to force myself to 'get out there!' And it worked.
Turning off the telly, living off-line more and more (cause I still spend a lot of time on these forums) are, to me, ways to live the 3D experience more fully.
Of course, any reason to leave FB is valid for the person involved. No need to defend your choices in life, really.