Seems like dedicated social networks is getting a lot of attention.
I like http://www.ning.com/. Here you can set up your own network without having to host it yourself.
You can set up groups, add gadgets, upload photos, add events and more…
My running friend Phil has set up this: http://planetultramarathon.ning.com/
Now it looks like http://elgg.org/ wants to be popular player. Can anyone tell me what it gives me more than ning?
In the wiki area there is http://www.wetpaint.com/. Also here you can set up a hosted community.
You can write pages. Each page has its own discussion forum in the bottom.
You can upload photos, add widgets.
An example: http://undocumentedgoogle.wetpaint.com/?t=anon