The data was first reported by Gamespot and comes courtesy of Valve's Steam Statistics page, which shows the exact peak figure was 7,190,547.
At the time of writing there are 4,097,728 people logged onto Steam.
Steam is Valve's digital game service, and the peak number of users came on the last day of the service's Spring Sale which featured heavy discounts of up to 90% off.
Steam's most popular title by far is Dota 2 with over 200,000 people playing right now. Starbound comes in second place with a very respectable 50,000 players.