I use facebook to link up with old friends and collegues. Now less and less, as not much happens on it anymore, as most friends I know that are likely to be on are now on it and have found all their likely friends. So now it is quieter than the rush of last year.
The novilty has worn off, people have already caught up with old friends, the funny apps have now worn off, they need another reason to check facebook so they need a big push to keep people checking the site.
The events organiser needs to expand perhaps, as only some people use it. Have it integrate with other tools and calendars (google/Outlook), merge with marketing do's, football fixtures. SMS notifications etc. My other suggestion is upTo soon to be at flurdy.com, an application for people to list what they would like to do. Not explicitly every time, but more "at weekends i am up for a game of football", or "fridays I would agree to go out for a drink". So when you think right I fancy going to a concert this weekend, you can quickly see which of your friends could be interested. A quick poll later and you got your gang.
Linkedin.com is good as well. Other people probably have had more use out it for creating business proposals, but I find it usefull as a career aid and whom to contact regarding certain situations etc.
doostang.com is very slick. More aimed at pure recruiting. More usefull for the american, californian people, however as its run by my old flat mate Mo, I am biased.
Today I found geni.com, very nice. Again a data privacy situation like facebook and linkedin, but I am already pumping in my family tree! Think they will quickly grow their users, as people have to put in family hence spread the network, as opposed to optionally spam friends on other sites.