Valve isn't giving up on single-player games, it's just adding social components to them.
A statement made by Valve for Geoff Keighley's Final Hours of Portal 2 iPad app made it seem like single-player games were on their way out at Valve. According to an interview with Valve head honcho Gabe Newell (done by a high school student), this isn't true all all. In reality, Valve will simply be creating games that are "single-player plus," rather than just single-player.
The statement was: "Portal 2 will probably be Valve's last game with an isolated single-player experience." Some took this to mean that either Valve wasn't going to make fully single-player games anymore, or that Valve wouldn't be dropping players by themselves into worlds with no NPCs.
Gabe Newell clears up the situation by saying: "What we're trying to talk about is the fact that, not that we're not doing single-player games ... It's more that we think we have to work harder in the future, that entertainment is inherently increased in value by having it be social, by letting you play with your friends and recognizing that you're connected with other people. That's the thing we're trying to say."
"Single-player is great, but we also have to recognize that you have friends and want to have that connected as well," Newell added. "It's not about giving up on single-player at all, we actually think that there are a bunch of features and capabilities that we need to add into our single-player games to recognize the socially-connected gamer. Every gamer is instant messaging, every gamer has a Facebook account. If you pretend this doesn't exist you're ignoring the problems you ought to be taking on, so it's 'single-player plus,' not 'no more single-player.'"
I'm glad we have that all cleared up then. The statement primarily impacted the future of Half-Life, Valve's most popular single-player franchise, with a highly-awaited next title that Valve has told fans to hang in there for. Valve changed the single-player game with the original Half-Life, so it would have been depressing to see the company move away from the craft.
The interview reveals that DotA 2, a very not single-player game, is likely Valve's next big release. In fact, Newell says playing DotA 2 has him "by the scruff of the neck." The entire interview, which also goes over Newell's knife collection, can be listened to below.