Three top game titles, including "Dota 2," helped Steam reach a record number of users on Sunday. Courtesy/Valve

Steam, the most popular online digital distribution, gaming and social networking platform, reached a record 8.5 million concurrent players on Sunday night. The strong start for 2015 was thanks to its top three titles, “Dota 2” (“Defense of the Ancients”), “Counter-Strike: Global Offensive” and “Team Fortress 2,” which all appear to have expanding user bases.

This isn’t the first time the platform, which is owned by the Bellevue, Washington-based Valve Corp., has reached 8 million users. Steam boasted 8,020,813 users on June 29, during its summer sale. It first reached the 7 million user mark in December 2013, days after its winter sale came to a close. Steam users appear to be growing steadily since it hit the 6 million user mark in November 2012.

Steam game “Dota 2” also reached 10 million monthly players. The 2013 multiplayer online battle arena game is increasing rapidly in popularity, thanks to positive reviews and frequent use in video game competitions. In July 2014, “Dota 2” championship “The International” took place in Seattle, crowning Chinese team NewBee as champions. The prize pool was more than $10.9 million, the largest award in the history of competitive electronic sports.

Steam's latest user numbers might have been boosted by PlayStation Network and Xbox Live being taken down over the holidays due to a number of DDoS (distributed denial of service) attacks. Hacker group Lizard Squad took responsibility for the attacks, which led to problems with both networks that lasted nearly a week. Steam faced few such issues over the holidays.