Perhaps it is partly a generational thing Olivia, as I certainly don't know very many people under the age of 25 and possibly 30 who are not on it.
My concerns and reasons for not having it are:
I'm a private person and don't want people seeing pictures of me or reading information about me (I know I can set high privacy settings, but still)
I really don't know why anyone would really be interested in reading about what I'm up to anyway - have they really nothing better to be doing than finding out what I had last night for my tea??
I don't care about reading about the details of other people's lives.
There's an expectation to update regularly. I couldn't be arsed with that.
What if my opinions and views change from what I posted a year ago? People can still read my historical posts and might not realise I don't particularly care for Gangham dancing anymore...
It takes up a lot of time for some people. A lot!
There's pressure to get likes and people feel unpopular if they don't.
People (even subconsciously) compare themselves to others and feel disappointed if their life does not match up as well.
It promotes superficial friendships that are pretty meaningless in RL.
People are sucked into advertising for companies - I don't want to be their lackey just because I might win a year's supply of tomato ketchup.
I'll stick to MN instead thanks! 