Yesterday, Steam reached an incredible new high whilst managing to have 14.2 million concurrent users online. The previous record was 13.9 million concurrent users on January second.
At the time of the record being broken, DOTA 2 with 951k users, followed by CS: GO with 675k and GTA V with 116k users.
At the time of this article, there is currently 11.9 million users using Steam. You can view this HERE.