Yep, it's a friends and family thing. Often friends of friends. Anyone who gets to know me a little bit can get an account. Since growth is controlled, costs are low. Since users are expected to donate, that completely stops being a concern.

The world is a smaller place than it seems. I've noticed this site having a measurable impact on the web around me, and lots of files that might have disappeared are sticking around now (23,946 of them, in fact).