Seems extremely inefficient to me being that the platform is socially and non-business arena. Basic economic theories.
I would argue that there are those that use facebook for purely social reasons and those for purely business networking reasons. However, given the format of Facebook, such networking is cloaked in a social uniform. But if there were no business reasons to join Facebook, it would probably be quite a few million fewer people joining up( Yes, I picked that number out of the blue) Even more so with Twitter.
I dont see any reason why offering a domain is any more commercial in nature than ads on facebook offering memberships to men having dates with well endowed women .
However, why pay for an ad, when you can soft sell to your network of "friends" - essentially networking with fellow domainers on Facebook like they do here on this forum