Confession time: I know a lot of gamers. I know of a lot of people who play games (both on and offline) and most of them are very well adjusted and have friends. My experience seems to go in line with a new study from Penn State, who sent out questionnaires to 150 video game players. The questionnaires asked about their relationships, and the conclusion was "Not all video game players are destined for lives filled with failing relationships and dwindling friendships."
Was anyone seriously worried that the opposite might be the case?
I know playing video games is in and of itself not very social in the traditional sense, but it can help bolster the confidence of a shy person, or get someone more introverted involved in a common social activity, especially if the game has a forum that people can chat in or is a multi-player game. I know that playing video games frequently is not a good way to interact face-to-face with friends (and yet another reason why I started the Friendship Boot Camp), but it can help you learn about relationships on some level.
I don't think it's the activity you do as much as the way you handle yourself overall that affects your friendships. Anything taken to an extreme can be bad. So if you're a video game player, you can use the online friends you meet to enhance your life. You could even meet a friend in person that you were first introduced to online like that.
But like anything in life, balance is important. The bigger lesson here is that online activity isn't a bad thing unless it isolates you.