I'll discuss some of my personal experiences with Web 2.0. (See Tim O'Reilly's article for an in-depth discussion of what Web 2.0 means.)
Well, this here is a blog! Blogging is a great way for me to practice writing and expressing myself. If you're a really popular blogger like Tim Ferriss, you can have very lively conversations with your internet audience. No one really talks to me about my blog. Not that I really expect anyone to, given that I write about random topics of personal interest and make no effort to connect with a target audience. But if you want to communicate with people via blogging, you can definitely make that happen. Even if it's on a small scale like sharing infant photos with the family.
I really like using Blogger because of the tags, anonymity, and free storage space.
I like Facebook a lot. I will frequently double-post on Facebook and Blogger. I post on Facebook to let my friends know about a link I like. It's much less intrusive to do so via Facebook than via email. I also get faster feedback. When people log onto Facebook, they know consciously that they are having fun and blowing time. They are in the mood to respond. Facebook is viral in the sense that when people see one or two comments to a posted link, they themselves feel compelled to join in the discussion. In contrast, email is this awful catch-all for communication. My email is probably sandwiched in between a reminder about a meeting and a message from the boss. I also post on Blogger so that I can have a personal record for myself.
The main difference between Facebook and other Web2.0 tools is the sophisticated privacy tools on Facebook. So depending on the user, Facebook is not as "open" as other platforms.
Photography is only a minor hobby of mine, so I don't spend too much time on Flickr or not as much time as some people do. I use Flickr pretty much the same way I use Blogger, to keep personal records that happen to be public. The main reason I chose Flickr is the excellent interface for tagging and viewing photos.
Sometimes I'll get some nice, unexpected comments on my (amateur) photographs. That's the magic of tagging and geotagging on Flickr. It's so easy for people to find photographs of a specific location or specific tag. Two of my photos have been chosen for the Schmap online travel guides. I think it's because I did such a detailed job of tagging my photos. There are probably better photos out there, but mine were easy to find. I'm glad to pitch in my bit to make Web 2.0 the great resource it is. Flickr allows me to explore other people's interests and photographic creativity. Many photos, including mine, are licensed under Creative Commons so they can be re-used for non-profit purposes. I'm not too familiar with the social aspects of Flickr, but it appears there's quite a community there. You can join all sorts of groups. I was asked to submit photo to a Sharpie pen group!
I'm a member of the ModSquadHockey forum and I used to hang out a lot in musical forums like Finishing the Chat and BroadwayWorld. Reading forums is a fantastic way to quickly acquire knowledge for a new hobby. I figured out how to pick my own hockey equipment and learned what theatregoers look for when they attend a play or musical. I'm still a member of the hockey forum because I really liked the atmosphere on the board (vigilant administrators are a must) and I periodically answer questions. I try to give back.
Wikis can be interactive but not on the level of blogs, Facebook, Flickr, and forums. The interaction is with the members of the wiki who write and modify the contents of the wiki. My only experience so far is submitting reference cards to the Starting Strength Wiki. I did get a nice note from the Starting Strength Wiki administrator, praising my contribution. I'm interested in doing more with wikis but I haven't gotten around to it yet.