Social Media Marketing

I'm thinking about reactivating my facebook page because i'm hearing it's more and more used in business and profession worlds. And it's great for keeping in touch with old contacts. But, i'm worried about the time cost vs benefit. What or the pros and cons to having a facebook page or anything similar to facebook.

Also, what are the pros and cons of it? If you can't interact or keep up with say, 500 friends or so on Facebook, what's the point?