Google Plus seems to be exhausting its novelty value, and is apparently experiencing a sharp fall. It's still in beta though, and not quite complete (as the article says, business users are underserved), but Facebook hasn't been sitting still as indicated by their recent addition of features inspired by Google+. This may still be a very uphill battle for Google.

Meanwhile, StumbleUpon is actually gaining ground it seems! It shouldn't be forgotten as a potential traffic source, especially considering it is all about link sharing. I believe sites in StumbleUpon also get seen more depending on how many users upvote the site, and they do so only if they find it really valuable. This might mean that the StumbleUpon traffic might be directly proportional to the quality and value of the site.

How's your G+ and SU traffic?