After the anemic response to Windows 8, Microsoft needs Windows 10 to become a massive success. Now that the operating system’s been on the market a few weeks, how’s it actually performing? According to a tweet from Microsoft executive Yusuf Mehdi, Windows 10 currently runs on 75 million devices worldwide. That comes two days after The Register (citing StatCounter) said that Windows 10 had 5.95 percent of the desktop OS market. That “75 million devices” statistic comes with some significant caveats. For starters, Microsoft is offering Windows 10 as a no-cost upgrade for current Windows 7 and Windows 8 users, a move designed to spike adoption at the outset. Second, Mehdi’s tweet offered little insight into how many of those devices were PCs, as opposed to tablets or smartphones—while Windows continues to dominate the desktop OS market, Microsoft has struggled to make any sort of headway into mobile devices. Third, given the number of copies that Microsoft's giving away, it's difficult to determine how much money the company's actually earning off the OS. Fourth, analysts estimate there are more than 1.5 billion PCs in use worldwide, meaning that Microsoft still has quite a long way to go before Windows 10 controls a majority of the market. It will be several more quarters before any sort of trend in the platform’s adoption becomes clear; and there’s always the possibility that Windows 10, like Windows 8, could end up struggling mightily to overcome the market-share for Windows 7, the company’s last blockbuster operating system.