I have used Flock before back when I was in my social networking phase. In fact, I was a somewhat active member of it's community during their early days. Never really got into the scene, though, so I dropped it after a while as I didn't really find much use for most of its features. If your wondering what Flock offers, well, it offers a lot. If you're the kind who does a lot of blogging, uses YouTube and Flickr, participate in social networks like Facebook and MySpace, then Flock is probably the ideal browser for you. You get things like media streams for your favorite Flickr and YouTube watchlists (and probably more services besides), a built-in blog editor that is easily integrated with Flock's other services (eg. drag and drop an item from your media streams into your blog post), a better news feed reader than Firefox's default (IMHO), and Flock's My:World page that integrates all your services together. There are a lot more stuff worthy of mention, but I can't really list them all (and since it's been a while since I last used Flock, I'm not sure I even know them all). As for Flock's popularity, keep in mind that it was built for a niche market (social networking) so many people outside of that niche haven't even heard of it. It's still a pretty big browser for it's intended market, though. ^^
I have the latest version 2 beta which uses the same code that firefox 3 uses. I love the default theme of flock, but thats about it i've heard about flock years ago as well.