Do you remember Venn diagrams from high school math? Those are the overlapping circles which represent where two groups intersect. Social media circles have started to intersect as well, and keeping them apart may not be as easy as it seems.
If you're wondering why you may want to keep your social circles separate, consider this scenario: you had a great Saturday night with your friends, maybe drinking, and someone might have put some rather awkward pictures of you on their Facebook account without you knowing. By tagging the pictures with your name, your boss could find this picture on Monday morning. There have been an increasing number of reports of people not getting jobs because of what potential employers find on social networking sites, and at least one person I know personally has been spoken to about what they've posted on their Flickr account. It's become quite difficult to be yourself on the Internet. Who you are outside work might not be who you are at work. If you already have a job where they know you well it may not be an issue, but if you're looking for a job, drunken pictures of you may not be the best thing to have floating around on the Internet.
Your workplace may not even be the only group you want to keep separate from your social network. It's possible you may want to prevent people you know personally from seeing that you're a hardcore gamer. Some people might not want to see messages of what your level progression in your latest gaming addiction is. This social networking "noise" could be another potential reason from separating your circles. People may follow you on FriendFeed or Twitter for some reason, only to see over time that some of the things you write about may become less interesting to them. I've personally had people unfollow me because either the gaming friends don't like hearing about social networking or vice versa.
How can you keep these worlds from colliding? It all comes down to how much work you're willing to put into keeping your worlds separate.
If you have a nickname, you could consider using that as your first name for one of your accounts. Facebook frowns on people not using their real name, but if your real name is "Bob" and your nickname is "Freddie", you can get away with it more than if your nickname is "Tbone". Still, it takes a lot for Facebook to delete someone's profile based on whether or not they're using their real name. With sites like MySpace which doesn't care what name you use, you'll have a better chance of keeping the account with your nickname, but as you get older, the appeal of MySpace may start to dwindle.
If you do use two accounts on Facebook, make sure that they don't have the same picture of you. A search for your last name would result in both IDs coming up in a search, and if both accounts have a picture of you on them. If your last name is unique, it might be a good idea to keep the area you're from vague, like "Northern California" instead of being more specific like "Sacramento".
LinkedIn is a great resource for professional contacts. By planning ahead and keeping personal items off your blog, you can keep a resource of your work online while keeping your personal blog separate.
Twitter has exploded in popularity. The downside as I've stated before is that the noise a person makes can push people to unfollowing you. Also, there's no way to post private messages to multiple people. You can do it on other sites like Plurk, but asking people to join another socnet might not go over so well when formerly popular ones like Pownce are closing down. What you can do is create a second account set to private, and only allow trusted people to see it. Third party Twitter clients like Twhirl allow you to use different windows for different accounts, so that you can use multiple accounts much easier than if you had to use a client that only allowed one account at a time.
If you have an account on a picture sharing site like Flickr, it may behoove you to set the level on some of your pictures above "public". Flickr, and other picture sharing sites, have different levels of security so that people can't see all your pictures unless you allow them to. Flickr uses "friends" and "family", in that order. Personally, I'd rather have groups instead of strict levels. I might not want Aunt Midge to see all the pictures from a lower security setting. However, even though you have the pictures set to a certain security level, that doesn't prevent someone from taking a picture off your site and posting it to theirs with your name attached to it.
If you have a large group of people that want to share a forum with pictures, you may want to consider getting a ning group at ning.com. Their service is free, and you can set the entire group to private, sending out invitations to only those people that need to see the content on the site.
The last piece of advice I can offer is to just be cautious of the information you post about yourself on the Internet. Regardless of your personal feelings on whether companies should screen candidates by searching for their name on the Internet, it's being done and the best you can do is just make sure that they don't find anything that can hurt your chances of getting hired. Keeping circles separate might seem like a chore, but in the long run, it could save you a potential problem in the future.