Ah, there is no Source Engine version. it's still HL1 /Gldsrc, there is a beta, but you have to be selected to be a tester, and they are full at the moment. No release date, but I'm hoping before 2013 myself.
You downloaded 1.2.3, right? IF you downloaded 1.3 beta, then that makes sense, no one plays that. Also, it's not like there are people playing it at all times, but you know, maybe change up your hours abit and you'll find people playing, I never have an issue, though I rarely play these days, cause I didn't play for so long, that I lost any tiny fraction of skill I might have had...and the only people still playing utterly dominate me in a way that the game just becomes non-fun. It's not like I can even train to get good, as there are rarely any new people.