LinkedIn is the premiere social network for professionals. I've been a member for about a year now, and while I haven't discovered all LinkedIn's possibilities by a long shot, these are the things I have found enormously helpful.
1.
Reconnecting with old colleagues. LinkedIn tells you which (current and) former colleagues are in the network and when new ones sign in.
2.
Reconnecting with old classmates. Works the same as with colleagues, drilled down by years of attendance for your convenience.
3.
Job search. Lots of quality job postings, PLUS, you can see who in your own network is hiring. A fantastic tool for employers and job seekers.
4.
Establishing credentials. You can recommend members and be recommended by them. You can display your recommendations on your profile - very helpful indeed! (Your personal profile is similar to a resume, only more robust.)
5.
Getting answers. LinkedIn recently added an Answers feature which I really like. Members ask business related questions and members respond. Questions are broken down into categories for convenient browsing. I've stumbled across several questions I had myself, and was able to read 10, 20, 0r 30 replies from experts in the given field. Quite powerful. I've also answered a several questions and received "Best Answer" recognition, which helps establish credentials.
7.
Making new contacts. That's the whole point, right? Your contacts can introduce you to their contacts - an ideal way to meet peers, potential employers, and potential clients. In the LinkedIn world, it's hard to contact someone without that personal connection, making LinkedIn introductions high quality.
