However in recent months, LinkedIn has this new "profile update" feature where if you aren't careful, everybody in your address book who is not already connected to you will get a LinkedIn invite. This can be disconcerting if you have a lot of people in your address book who are not work colleagues because they you will have aunts, cousins, casual friends and whatnot getting these invites and they will think you are spamming them.
Like you, I use it to keep track of colleagues both for recruiting and potential job-hunting purposes. I have FB but only use it for keeping track of family and closest friends. As much as I hate both sites, they have enabled me to touch base with peoe I thought I'd lost contact with forever. Sad that they have to use it for data mining but they have to pay the bills.