AOL Instant Messenger (AIM) is an advertisement-supported proprietary instant messaging and presence computer program which uses the OSCAR instant messaging protocol and the TOC protocol to allow registered users to communicate in real time. It was released by AOL in May of 1997. Stand-alone official AIM client software is available for Microsoft Windows, Mac OS, Mac OS X, and Linux. The software has a large share of the instant messaging market, especially in the United States (with 52% of the total reported as of 2006), placing it ahead of the combined total user base of the two closest rivals as far as the U.S. market is concerned. It is maintained by AOL LLC.