Steam quietly welcomed its billionth registered account in April. Steam ID Finder shows that they joined on April 28, netting themselves a Steam ID with a lot of zeroes, but no parade. Valve hasn't marked the occasion, probably because the number doesn't mean quite as much as concurrent or monthly active users. Both of them are still pretty high, too.
The problem with using registered accounts as a metric of success or popularity is that some of these users will be banned, bots or already have one or more accounts. The concurrent figure paints a slightly clearer picture.
Last year, Steam's concurrent users peaked at over 18 million, in some part thanks to the massive success of PlayerUnknown's Battlegrounds at the time, and since then it's been hanging around the 15 million mark. 2018 also saw Steam average 47 million daily and 90 million monthly active users, a 23 million jump over the previous year's monthly figure.
Possibly the largest factor in its growth spurt has been the rising popularity of PC and Steam adoption in Asia, particularly China. In 2012, sales on Steam were completely dominated by Western Europe and North America, with no other region coming close. Six years later, and despite the fact that a lot of games don't have Chinese language support, and the Asian market, in terms of game sales, was almost as large as North America.
Whatever figure you look at, Steam's still gargantuan and all-consuming, though now it has some competition and exclusives on other stores to contend with. That won't change the number of registered users, and Steam still has hundreds and thousands of games, but with more publishers opting to take a different route, the concurrent player numbers could conceivably see a dip if it continues, though there's yet to be a noticeable impact.