Steam is certainly a popular service for PC and regularly features millions of concurrent users. However, in a sudden spike of traffic, the service passed 9 million concurrent users.
At about 9:03am today, Steam reached a record breaking 9,264,841 users logged in at once. Before then, the service was a bit over 7 million and afterward alternated between 7-8 million before settling on the current 7,485,608:
While the reason for the sudden traffic spike is unknown, I wonder if it represents an eventual level of concurrent users that Steam will reach in the future.