Image: Ellis Hamburger, Business Insider

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The rumors were true: Google will charge a much lower commission rate than Facebook for in-game transactions.

But only during a limited-time promotional period.

Google+ managers Punit Soni and Bradley Horowitz told VentureBeat that Google will take 5% of revenue from in-game transactions. They didn’t say how long the promotion would last, but it could certainly get a lot of social game developers to give Google’s game platform a shot.

Facebook takes 30% through Facebook Credits, which became mandatory for all in-game purchases this July.

This is how Google often plays in new and vital markets — by undercutting the competition. Look at Android for an example.

Maybe Facebook will have an answer in its game developer announcement later today.