Having joined Facebook and Ning and having looked at Delicious I appreciate how useful these could be.
Ning - really useful for groups eg clubs or a work project groups or colleagues not at the same location.
Facebook - this is not for me at the moment. I'll have to wait for a "need" such as Wendy in keeping in touch/up-to-date with family and friends. For me, I don't like the "publicness" of it. I even used a madeup name.
Delicious - I think this is useful for those of us who share computers at work.
I have to say that there are huge possibilities but I like my old-fashioned system of email alerts (rather than RSS - even for database searching!), using email (own domain name) for most things plus favourites on home computer. Might have to think of a better system for work though. The possibilities are endless...