The first sentence of the release:
"has surpassed 13 million active accounts."
7 billion player minutes a month divided by 13 million accounts is 538 player minutes a month per account. It seems plausible to me that most of those accounts really are active.
My steam account is still active, because I'm still playing Counterstrike every once in a while. I still install the occasional single player mod for HL2 (the Awakening was quite good, really), and Garry's mod is addictive and I don't know why.
Steam has quite a bit going for it, despite all of the hate.