It's my understanding that as long as the content does not infringe then you may be safe. Also Facebook and other Social Network sites actively encourage third party development of gadgets for their sites as it adds to the appeal of their site to have these gadgets available. I think it would really depend on what was done with a site associated with that domain name and just how sue happy the trademark owner is. Some companies just spam the world with cease and desist orders that are sometimes nonsense (IOC telling Olympic Coffee House in Washington State to stop using Olympic in their name which was a reference to the Olympic mountains where the shop was located, for example) Meanwhile MySpace has never made a move against MySpacePros.com for trademark infringement.