People just tend to tell people of things they enjoy. You know what you're doing when you make a thread about a game? You're advertising that game. There are two reasons that people post these threads. One, to have fun, these are the singleplayer games. Two, the advertising, or the "come play this game". Those are the multiplayer games, that is advertising for more people to play it, and as a get-together for people who play it. Ultimately even mentioning a game is advertising it. Chances are that every thread you make in Other Games is going to get at least one person to buy it or try it in the case of free games. In fact, for free games that have donation options, you're going to get at least one person to donate as well. It's just the way it is.
Steam is more than a game store though. It also has a community. It makes it easier to connect and talk to friends and fellow gamers. People are going to advertise communities because they want more people to join that community, so that they have more people to play with and talk to. A community has to expand somehow, and that is mainly by word of mouth. How is Dwarf Fortress so popular, when it's hardly had any media coverage at all? Word of mouth is what gets games a large fanbase. Large companies are different though. They rely on older games, and actual advertising. The thing is, there are plenty of games that have gotten by purely on word of mouth. Ever hear of Darkfall Online? Sure, it's not that great, but there was a huge fanbase behind it at one point. There still is. Aventurine did nothing to promote it. Pure word of mouth.
The point is, people want to play with other people. Even if it's a singleplayer game, like Dwarf Fortress, you get people coming together to share their stories. That's the essence of almost every game, is the community. The reason that Dwarf Fortress has this many people is the creativity of the players, and the ability to use this creativity in-game. Look at singleplayer FPS games. There are very, very, few. I can only think of a few, such as HL and HL2. This is because that singleplayer FPS games need to be really, really, good to be big. HL2 doesn't even have a large community. The main community for HL2 is in the mods. Again, it's creativity at work. Can you think of a successful singleplayer FPS games that don't have some element of creativity? I can think of a few, but most of the ones I think of have RPG elements. That's the element that makes most FPS games successful. Mass Effect is popular for both being a good game, and the RPG elements. Bioshock is popular too, and it has no mods, multiplayer, or RPG elements, or at least limited RPG elements from what I've heard of it (haven't played it myself).
People just like expressing themselves, and find that easiest to do with other people. You can't be very creative without somebody to share it with. The reason people share these things is ultimately for purely self-centered reasons. Just think about it. You share a multiplayer game because you want to play with other people. You share a singleplayer game because you want ideas to mess around with, like traps and designs in DF. There is no thread for games such as Bioshock, singleplayer racing games, and many others, because they have limited room for creativity. The player is limited in their choices. Games such as TF2 get massive megathreads from people talking about their experiences, and organizing games. Games like DF get entire forums, or threads on other sites, because people share ideas and designs. You share an idea and design in hoping to get some other ideas and designs back.
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