The Diaspora team have no issue with people setting up fansites. In fact I still drop by the BtRL fansites like BSGPilots whenever I can. What we would have an issue with though is you setting up an official Diaspora Facebook page for us.
The issue is simply that on a fansite anything you get wrong is simply something a fan got wrong, but anything on an official page is going to cause us issues with people saying "But your Facebook page says...." Furthermore there is always the ever present issue of fans who start something and then never finish/update it. A Diaspora Facebook page would need to be updated to match our website and ModDB entries. It's going to look pretty bad for us if someone searches for Diaspora on Facebook and found an official looking page that hadn't been updated for 2 years. I'd rather they found nothing and moved to Google than that.
So in short, we've got no problem as long as you make it clear you're a fan and can't speak for the team. The same goes for any other social networks people think we should be on.