It's certainly easy to measure the popularity of multiplayer gaming, and measure the decreasing trend in single player games.  One of the most popular games today, World Of Warcraft, is massively multiplayer, and hit a two million-person landmark in accounts logged.  An easily accessible, colorful world based on an award winning franchise always helps, but the question of its popularity is invariably answered by the copious amounts of players actively seeking to participate with other players, or kill them.

Whether acknowledged or not, the trend of the average gamer is to actively seek out others to play with rather than to test his mettle against the average drone, programmed by simple code to help or kill him.  If it were just the competition players sought, then indeed multiplayer would be scuttled in favor of smarter AI alone.  However, the company of like minded individuals is the opiate of the gamer, but what specifically fuels this desire?

Repeatedly I've asked this question to my friends, family, and other gamers I play with, and the resounding answer is the companionship found in the solidarity of gaming.  Whether cooperative or adversarial, a company of friends heightens the enjoyment of any specific game, whether it be Counter-Strike, City Of Villains, or Super Mario.  Obviously, this is just a generalization, meaning there are exceptions, but largely the people I've interacted with in the gaming community express a similar sentiment, that they want to enjoy the time they spend playing with their allies and opponents.  Clans, Guilds, or even supergroups are created for the sole purpose of finding friendly like minded people in any sphere of gaming, not to frag the nubz0r or whatever l337 prospects the army of 13 year old gamers may garner. 

Communities within games have largely helped or hindered the successes or failures of a given game.  There are countless stories of Counter-Strike players quitting because there were far to many angry players, or joining Everquest because they met a great guild online.  They go both ways, but in the end in a multiplayer arena, its not the thrill of the kill that keeps people coming back, but rather the camaraderie found with the people enjoying a similar hobby.