changes in content frequently will help to attract crawlers to visit any website again and again. Forums. blog. and SB sites are following the same, so that's y they got this power in eyes of Google.
I also remarked that this go too with a joomla website. It index itself very fast and you got an alexa rank around 3 500 000, wich is very big for a site that haven't been promoted...
I have a Wordpress blog (not hosted on Wordpress, tho) and as soon as I post a new article, I immediately see both the Googlebot and Technorati bot show up to index the changes. My new pages show up in the SERPS almost immediately. But I still don't have any PR (even with 6000+ quality links & site's > 4 mos old) , so I'm not quite sure what's up with Google. Still sandboxed to some degree, perhaps?
Yes, the social bookmarking sites themselves are indexed rapidly. I added a new page to my site yesterday and also added it to some social bookmarking sites. A Google search for the file name already shows two of those SB sites indexed for my new page. But that does not necessarily indicate that the search engine went on to crawl my page. In fact, my page does not show in the search results yet, but two SB sites listing that page do show.
The fast indexing is observed with the sites that are frequently updated like news sites, blogs, social bookmarking sites like digg etc.
I have noticed that Digg can get your pages indexed fast, but after a few days, the pages then disappear.
With the amount of new content every day it's normal that site like Digg got indexed in no time. The google bot is not lazy
it's pretty handy if you are waiting for google to index you, and the social bookmarking site link is already listed, might get you a bit of traffic while you are waiting for the google bot to do its stuff.
I think Google believes it is user generated content and also the moderation of what goes to the main page is also decided by users. So Google places a greater trust on social sites and they rank very well.