Blogs - writing; single person to mass audience. Singular feedback to/from blog owner only.
Facebook - gossip and photo sharing; single person to mass audience
with sharing. One to one, one to a few, one to many - with feedback to/from others, and ability to form (and break) relationships.
(The games keep a lot of people on Facebook too!!)
Twitter - short-form gossip; single person to mass audience with limited writing space, which creates a more succinct message that is easily read and digested. "A quick way to keep abreast of things that matter to you".
Forums - knowledge sharing; multi-person discussion around specific subjects that allow people to share their knowledge and experience with others, and form online (and offline) friendships with people of a shared interest/passion. Offers a greater depth of discussion that you don't find on Facebook or Twitter, and with more interaction than simply adding comments to a blog.
Forums FTW!!
Cheers,
Shaun