I'm a fan of LinkedIn.com. To many people it may look like a waste of time, or maybe a moneyspinner, but i've paid nothing and found it useful in encouraging me to build contacts with other techies in Brighton + i've been able to ask specialists in e-learning and Pisces XML for information. In fact, I just got my new job through it. I had seen a job advertised on jobserve, but luckily the agent was lazy enough to cut and paste the job description rather than understand the client's needs and rewrite it. Google didn't throw it up, but then I remembered the job section on LinkedIn and found it there.
The approach that they've taken for their interface has influenced my thinking. Little things like the slider gauge that says, "your profile is 90% complete…get another recommendation" are a subtle way to encourage people to use all the features. New features are released without a fanfare because if the design is right then it should 'just work' (one of my other influences is Donald A Norman who says, amongst other things, that design should 'just work' and that if you need instructions then it is designed wrongly) and, like with Google, it is often a pleasure to discover new stuff for yourself.
The only problem i've had with it was the 'new job' notification. There is a drop-down shortcut for sending a message. Normally this takes you to another screen where you can compose a message. I clicked on this to send my contacts a notification, but no new screen appeared and I got no feedback whether a message had been sent or not. I've asked a couple of people whether they got an email and they said "no, but I might have email notifications turned off". This isn't too cool.
If it were my system, the only change I would make is the ability to discriminate between a 'contact' and a 'friend'. Some people abuse the system by collecting as many links as possible. I normally turn them down, but maybe I should be able to add them as a loose link until I know them better.