I meet a lot of people in my job, many of them are in communications, marketing, or media, some of them hate Facebook and/or Twitter with passion.

They typically say that they won't join because they don't believe in them. I typically respond that these aren't mythological creatures, Santa Claus, or a God that requires belief.

They don't require a highly emotional response. You don't have to like them. Or hate them. They are what they are. 

I tell them that you should be on them because you work in marketing, communications, or media, you need to know about these media channels.

You can't read about them, you need to be in them to know them, to see how people are using them. 

You need to know what's acceptable in those communities, what people are sharing and not sharing, what events they are going to, what they are saying to each other and how they say it. You need to know these things. 

You should be on them because if you are not then you won't know when things change, and they change constantly.

You don't need to be active on them, you can lurk.

You don't need to share what you had for lunch, or the other inane stuff you complain about that others share, or that you heard they share. Share good stuff or don't. But you need to be on them.

You need to be on them before others find out that you're not.