I know that I am very late to this thread, but I just wanted to say a few things. First off, the insinuation that PRS is simply here to "steal" members seems a little sour grapes. As an active member of a few different geek craft communities, I can honestly say that having a new forum to go to never stopped me from going to any other ones. Second, the reason everyone seems to know each other at PG is because the mods have gone out of their way to befriend everybody and promote friendships between the other artists. People have formed these bonds because they wanted to, but its up to each new member to decide if they're going to jump in, or just hang back. These types of bonds promote the popularity of the site and the amount of posting. I do not see this at SS.
Speaking practically, as a blogger, I can say that cross promotion really is key. What's good for the goose is good for the gander, as they say.