Original PCGamesN Source
According to Steam's stats page the peak for today was 7,190,525 at once. Last year, on November 26, 2012, the concurrent peak hit 6 million at once. That's a growth of 1 million constant users over one year (plus a few days). This comes just one month after Steam reached the milestone of 65 million registered users, rising over Xbox Live for the first time. (Source: IGN)
I couldn't find any solid, recent numbers on the concurrent peaks of PSN and Xbox Live. If anyone has sources for those I'll add them to this post.