Why make things more complicated than they need to be? Avoid friending coworkers -- period. If you're concerned that you might offend someone, explain that your refusal isn't personal, but rather a matter of professional policy. After all, respecting boundaries goes hand-in-hand with respecting coworkers.
This might strike you as more confrontation than you want to take on, but the alternative entails that you either friend the entire office or risk offending every coworker whose friend request you ignore or reject. As an additional incentive, just saying no spares you the complications that inevitably follow from non-invites to posted events, broken office romances or company departures.
Again, remember that Facebook is not private. Matters discussed online could get back to clients or even find their way into the hands of reporters. Insults and opinions might theoretically take on the legal force of defamation, or at least get you in hot water with human resources. These factors alone make mixing business and Facebook a questionable prospect.
Besides, given the growing accessibility that work has to our private lives (with cell phones and the constant connectivity of mobile devices), don't we owe it to ourselves to take back a tiny slice of life that's ours alone?