RPI (roleplay intensive) games are basically 'always IC'. If you're logged in, people can interact with you. Coded actions are considered roleplayed actions (i.e, if I walk into a room and use the 'punch' command on @Arkandel and don't emote about it, I'm still ICly punching Arkandel). Limited in-game OOC communication, usually just a 'help' channel, a local OOC command, and something to communicate with staff. Out of game they usually have forums with user, not character, based identities.
MUDs and MUSHes are two sides of the same coin! I'd just take 'social MUD' to mean 'code heavy RP game', but @Streetwitch may have their own idea about what it is.