Most multiplayer games and platforms already have some successful social features. Friends lists, guilds/clans, and party systems are all examples of useful social design. I think everyone can agree that those social features definitely increase the fun of playing multiplayer.
But those features are not enough; they are really only valuable if you already have friends online. If the multiplayer games are going to be welcoming to new players, we need social features that affect newbie who may not (yet) have friends.
A useful feature that doesn't require friends is the swear words filter for text chat. Even well-moderated text chat channels get nasty. But when the swear word filter changes words deemed "offensive" ("asshole!") into something less offensive ("*%@^!"), the reaction to the swearing drops dramatically, because this change makes the swearer look... well, silly.
Amusing, like a cartoon character, rather than aggravating. I'll bet some newbs don’t even realize that the swear word filter exists until they type some choice words themselves and see their own chat "sanitized".
Another example of social design comes from Shadowrun, a team-based multiplayer shooter I worked on. One of the fundamental problems of team-based games is that some players (especially shooter players) aren't naturally team-oriented.
It is fairly common for team members to focus on beating teammates for prestige (high score, kills, scores, etc.) rather beating the enemy. Pre-pubescent voices often scream "kill stealer!" at a teammate during a team deathmatch, because they are focused more on their personal stats rather than winning as a team.
In order to reduce this kind of anti-team behavior, we changed the stats system to reward pro-team behavior over individual success.
We made the rewards for killing enemies proportional; so if one player does 90% of the damage and a teammate steals the kill, the first player gets 90% of the credit (and money!) for the kill, and the stealer gets 10%.
On several occasions, I heard players new to Shadowrun complain about teammates stealing their kill stealing, only to have team members tell the new player something to the effect of "that's not how this game works".