Ok, i'm not part of the core developers so the following is just the personal perception:
Right now i wouldn't say it is active neither dead, it's just that Superpowers since it is not a commercial product (meaning they not have a profit directly from it) it cant be maintained in the same way of other engines (like Unity), but on the other hand as an open source project anyone can clone its code and continue working on it and with that in mind i wouldn't say its easy to kill (but not imposible).
About multiplayer, Superpowers has a socket.io client library built in, so the hardest part should be how to setup the server.
If Superpowers is a good engine or not depends on your need, the only scenario where i wouldn't recommend Superpowers is if whe where talking about 3D games and i will fault webGL for that.