Steam is a digital distribution platform developed by Valve Corporation, which offers digital rights management (DRM), multiplayer gaming, video streaming and social networking services. Steam provides the user with installation and automatic updating of games, and community features such as friends lists and groups, cloud saving, and in-game voice and chat functionality. The software provides a freely available application programming interface (API) called Steamworks, which developers can use to integrate many of Steam's functions into their products, including networking, matchmaking, in-game achievements, micro-transactions, and support for user-created content through Steam Workshop. Though initially developed for use on Microsoft Windows operating systems, versions for OS X and Linux were later released. Mobile apps with connected functionality with the main software were later released for iOS, Android, and Windows Phone devices in the 2010s. The Steam platform is considered to be the largest digital distribution platform for PC gaming, and was estimated by Screen Digest to have 75% of the market space in October 2013. In 2015, users purchasing titles through Steam or through Steam keys from third-party vendors totaled roughly $3.5 billion, representing 15% of the global PC game sales for the year, based on estimations made by the tracking website Steam Spy. By late 2017, the service had over 150 million registered accounts, and had reached a peak of 17.5 million concurrent users. The success of the Steam platform has led to the development of a line of Steam Machine micro-consoles, as well as the SteamOS operating system.