If you look at the “Top Sellers” tab on the “Free to Play Games on Steam” page, you’ll see Warframe at the top of the list. It’s ahead of Valve’s own Dota 2, and if that’s indicative of relative sales — in other words, if it means Warframe is making more money than Dota 2 — that’s really saying something. Big as it is, Warframe only has about 15% the concurrent players that Dota 2 has on Steam, so for it to surpass the MOBA in sales is outstanding.
Here’s where all the caveats kick in. I’m not 100% convinced that the order of games on that “Top Sellers” list is a strict and accurate ordering. If you look at the “What’s Being Played” tab (which has “ConcurrentUsers” in its URL) and compare that to the actual list of what’s being played, you’ll see that it matches up … for a while.
I pulled these rankings just after the top of the hour, when (I think) Steam updates them. The What’s Being Played tab looks right up until War Thunder, and then it lists Smite, which has several other F2P games ahead of it on the stats page. Maybe the two lists aren’t updated at the same time, and the lists did match up, say, around midnight? Or maybe the tabs on the F2P page are more of a general listing, rather than an exact one? It’s hard to say.
In any case, Warframe is undoubtedly one of the best-selling F2P games on Steam right now. Whether it’s #1, and will stay at #1, is up for debate.