I've only been playing with Facebook for 8 months, but I'm adding five types of friends.
a) 50% current friends & acquaintances b) 5% current work/networking contacts c) 30% old friends & acquaintances 1981-2002 that I lost touch with (college, grad school, work) d) 10% my invites: people I've never met in person, but I found their internet presence interesting...or want to contact them in real-life later. e) 10% people who found me on Facebook.
On LinkedIn, I have 8 contacts of another type f) 7% ultra-networked people (20,000-40,000 friends)
Due to the way LinkedIn shows information, it helps to have at least a 3rd degree connection when trying to view someone (or to pay $240/yr). Connecting to a few ultra-networked people allows me access to 5+ million out of the 23 million people.