Song of Ice and Fire RPG has a great social combat system. Star Wars FFG has a simple system too that I like.
Let's be really fair though: From D&D to WoD this hobby has seen many systems used for social challenge moderation available, but they are not only not used, but avoided.
And it's not just because of creeper roleplayers who will try to turn it into something rapey.
See, the only way to have a character effectively lie to another character on most MUs is to lie to the player. This is because a lot of players aren't okay with having to RP something that they know is a losing play. Little bits of meaning happens all the time to avoid it, and many of them get pissed when they find out they were lied to oocly (TotallyNotMetaGaming).
It's that sort of vice grip on controlling the experience of their character, up to and including the character not having the winning combination (and the OOC angst that comes along with it) that has kept many forms of social roll systems covered in dice and cobwebs on these games player narrated character showcases for years.
For those of you that love social combat, when your character loses, and that can be mature enjoy to enjoy social risk without making it all about penises? I love you. I love you dearly.