Yeah, the “social” in social game doesn’t really refer to whether or not it is multiplayer. In large part, it’s a combination of the origin of the genre (social networking sites) and the tendency for them to give you in-game bonuses for advertising the game or convincing others to play more often, as well as providing tools to do that.
It may not be perfectly descriptive, but it’s a term that’s well known and ONLY applied to these sorts of games. The example about RPGs is a good one (or about roguelikes, or whatever). Genre names quickly come to describe some set of commonly shared attributes rather than a layman’s expectation from the name, and these games clearly have the attributes associated with social games.
Also, MOST of the “multiplayer” games I’ve found on the site since the influx aren’t even actually multiplayer, which is the worst part. They are… parallel at best – it would be like calling Animal Crossing a multiplayer game. At no point am I playing with other people in the same, although we can sometimes interact in ways that change our respective sessions – but (for a real world analogy, probably inaccurate) just because I can send you mail doesn’t mean we’re living together.