Mainly because I'm just introverted enough... to not want certain people in my life to have easy access to me. I know of a few family members who are all up in everybody's biz on it, and I want no part of that gossip mill.
My wife tried it for a while, and a few of her old high school friends about drove her nuts with inane crap on a daily basis. She found she didn't have a lot in common with them anymore (bored stay at home moms, and my wife has a career), so she shut it down.
Also, there's the privacy issue. I've already turned my soul over to Google
, don't have another one to sell out on a different platform.