Are you sure? I’m looking at Steam statistics, I’ve spent more time with Far Cry 3 and Borderlands 2 since their release more than HL2+EP1+EP2 together till now.
I’m not saying they aren’t replayable, but they aren’t something you would play continuously.
Listen to Gabe, when talking about Valve’s approach and future of gaming, he talks about MMO and multiplayer games constantly. Also lately, he also talked about which are the most played, but don’t remember what he said.