I just made a thread on here. I then searched the title of the thread and it was indexed already. How?
I have not access here to see beyond the users online, but being mod/admin in another huge forum that does not longer exists, let me tell you that search engines bots don't visit often forums of DP size, bots are really here full time crawling here. That is how they index up-to-date threads, literally
I believe if any website has every second hundreds of new content like DP, Google will work on it full time too. Contents and especially new content is the king.
I posted something like that in other thread, reminding that my friend's husband had a forum at which over 40 Chinese robots were planted, and of course all the other search engine. However you can't imagine all what he has to get his forum de-indexed when he decided to revert all, in fact three years later he has not finished his de-indexing job, LOLZ