Valve Software publishes monthly statistical information gathered by its Steam gaming service. Now that Steam has been on the Mac for a few months, information on Steam’s use by Mac gamers is beginning to provide a clearer picture about who’s using the popular service. This is no surprise given the trends in Apple hardware sales, but laptop users figure prominently into the mix.

According to the results of Steam’s recently-published July survey, MacBook Pros make up the single biggest percentage of Valve’s Mac customers – almost half of the Mac users on Steam have MacBook Pro systems. Another quarter of users use iMacs; MacBooks, Mac Pros and Mac minis make up the bulk of the rest of the systems, with a mysterious 0.74 percent of “other” systems running the Mac version of Steam. (“Hackintosh” PCs, perhaps?)

The vast majority of Mac gamers were using the latest build of Snow Leopard available at the time – 10.6.3 – and more than half had at least 4GB of RAM installed. And most Mac users using Steam use systems clocked between 2.0 to 2.99GHz – less than 10 percent use faster systems. And not surprisingly, given its popularity on recent-vintage MacBooks, MacBook Pros and Mac minis, Nvidia graphics figure prominently in the poll’s video card description results.

It also appears that Mac use has slipped slightly since the service’s debut. A report published in June shoewd that Mac users made up more than eight percent of Steam’s users in the weeks following the service’s Mac launch, but July’s numbers show that Mac users made up just about 5 percent of the active user base.

Steam was released on the Mac in May. It’s a game download service used both by Valve Software – for popular games like Half-Life 2, Portal, Team Fortress 2 and Counter Strike Source – and also by third parties.