It is my policy to only accept Facebook 'friend' requests from people I know personally, but usually not from those I only know professionally. This policy is not meant to cause offense, or disengage me from the online world.
Recently I have noticed I am increasingly being asked to 'friend' someone on Facebook who I don't know personally. My usual practice with these requests (when the person appears genuine) is to respond back with a private message through Facebook asking whether or not we have actually met. Given how active I am on other public forms (e.g. this blog, Twitter, LinkedIn) some seem surprised to find out that I reserve Facebook for connecting with people I know personally. I had such a conversation earlier today.
One of the key reasons for this policy is that (for their personal safety) we have always required our (now teenage) kids to only 'friend' people online that they know and would count as a friend in the physical world (family also counts!). To demonstrate to them the importance of this policy I follow the same policy for my own Facebook, and insist on being connected as 'friends' with them. From time to time we are allowed to check each other's 'friends' lists against this policy - and we do so. My kids are surprised at how many people their Dad knows!
The other reason is that I also like that I can keep at least one online forum (currently Facebook) more private. I am publicly active on enough public online forums as it is!
What is your policy?
Please note that I am well aware that privacy concerns with Facebook are increasing and that the policy described above could therefore be a bit of a mirage. However, until there is an alternative online forum where we can find most of the people we know then the reality is that Facebook will continue to perform this role in our lives.