http://pinboard.in gives me a list of people to check out based on their bookmarking history. I don't know the details of the algorithm and there's no flexibility in refining the recommendations but I've found some interesting people to add to my 'network'. You can only see the username of the person. The ability to find out more about people and then contact them as Massimo suggests would be a good feature.
The Facebook feature that lets you create 'lists' of friends so you can share things with sub-groups of your friends is not subtle enough - as far as I can tell it only allows you to send 'messages' to people in a list - this can be intrusive as it ends up in their 'inbox' and also usually emailed. It would be better if you could also do 'wall' posts that can only be seen by a sub group of friends (e.g. rude joke for my friends to see but not family).
