LinkedIn is geared more towards business professionals where Facebook was created for high school and college students (however it has recently been repositioned for those of us out of school). Both sites have their benefits and they are the only two I’m currently using (I wrote a review of LinkedIn a few months back in my e-zine).
I recommend trying both out as they each have cool features. Michael Port recently did a podcast with some excellent advice about how to promote your business on these services. You can listen to the podcast on Michael’s blog.