Valve revealed at the Casual Connect conference in Seattle that Steam now has 67 million monthly active users. In materials sent to Tom's Hardware, the company also said that its game platform has 33 million daily active users and peaks at 14 million concurrent users each day.
Note that Valve is specifically talking about the numbers of active users--the company didn't just keep a running tally of the number of people who've installed Steam over the last few years. This distinction has become increasingly important for software companies, because investors were burned by companies that crowed about their user numbers even if most of those people didn't actively use the product.
Valve also revealed that it's had 27 million new purchasers since January 2016, with nearly 1.5 million first-time purchasers added each month. This means that Steam is both attracting new gamers and keeping its existing users happy. The former probably isn't particularly hard--events like the Steam Summer Sale make it easy to find new players--but the latter can prove challenging for any platform.
Steam is no stranger to challenges. Users have recently bemoaned a new gifting system that doesn't allow you to stockpile games to give away at a later date, for example, and reactions to the decision to replace Greenlight with Steam Direct have been mixed. (Complaints about the latter were mollified when Steam Direct's publishing fee was kept to a low $100, however, when it debuted in June.)
Valve has also made a bunch of changes to Steam to keep its audience happy. Highlights include requiring developers to share in-game screenshots on store pages, adding support for the DualShock 4 controller, and improving the way customer support requests are handled. Valve's also explained some of the challenges in making a platform for everyone and improved curation and recommendation features.
All things being equal, Valve's numbers show that Steam is getting along well. It's not like Steam is the only platform-slash-marketplace on the virtual block anymore, either, thanks to increasing competition from the likes of GOG and Twitch. If things continue like this, Valve won't have to worry much about Steam losing its... well, you know.