The Xbox One version of Rocket League has more than 1 million unique players, and it has built that community without giving the game away for free.
Rocket League became one of the breakout hits of last year thanks in no small part to its inclusion in PlayStation Plus, which allowed PS4 users to download the game at no charge for the whole of July 2015. As that period neared its end, Psyonix revealed that Rocket League had been downloaded 5 million times. Not all of those downloads were on PS4 - SteamSpy pegged its player count at 500,000 at that time, and Psyonix announced 1 million PC sales a month later - and some would have been purchased by those who don't subscribe to PlayStation Plus, but it's safe to assume that the offer accounted for the majority of those users.
In terms of consoles, though, the Xbox One release was different. It launched on February 17, so that 1 million unique player figure has been amassed in just three weeks without having to give the game away for free. At present, Rocket League's price on Xbox Live is $19.99.
YES! We have officially eclipsed more than 1 MILLION unique players on @Xbox One! Thank you very much for continuing to support us!— Rocket League (@RocketLeague) March 6, 2016