We spend a lot of our time at work, so it's only natural that we'll make a lot of friends there, as well as a lot of acquaintances who we politely pretend are friends, while not really involving them in our personal lives outside work.
Social media sites blur the lines between our work lives and personal lives, but also between our personal lives at work, and our personal lives outside work. You could be quite happy discussing what you got up to on a hen or stag night, for example, with your friends outside work, but find that if you include your friends at work, the office gossip mill means that everyone knows by lunchtime.
Before you start connecting with people at work on social media, stop and consider what you're going to end up letting them into. It might be safest to allow some people into a limited profile, and save the really juicy details for people you know you can trust.
Also consider using the options in whatever social media service you use to hide private details from the public. Otherwise co-workers who you don't want in your close circle of friends could still be browsing your information, via links from the social media 'Friends' lists of others who work with you.
However, note that, in general, employment tribunals treat social networking sites as public spaces, no matter how few Friends you have.