Discovered on the current test build.
The info blurb for parties in the tech/development window implies that parties provide a social fix (as well as do quests).
Neither of these statements are true, as while watching one subscriber from the point of login (approximately 7:00:00 in-game time) to the point of logout (approximately 19:00:00 in-game time), the subscriber went from 63 social to -91 social. Nearly the entire time they were logged in was spent with a party, save for joining the party and going home to log out.
That is a 12 hour gaming session spent almost entirely with other people that resulted in a drastic loss of social need for a Socialiser. I don't think this is by design.