To the best of my knowledge Steam passed 7.5 million concurrent users over Christmas but has seemed to peak around 6.8 - 7.2 million since.
Today I noticed they peaked to 8,020,744 concurrent users, today being the final "encore" day of the summer sale.
I'm interested to hear peoples view on what they think about Steam's growth to date, and what events / games could push this further.
For reference, here are the yearly figures that were shown at Dev Days (late January 2014)
Edit: For reference, comparison of gaming accounts:
Google Play Games: 100 million - in 6 months (June 2014)
Steam: 75 million - 30 day active users (Jan 2014) / 186 million total (Arstechnica Crawler estimate April 2014) [65 million 30 day active Oct 2013]
PSN: 110 million accounts (undefined - July 2013)
Xbox Live: 48 million accounts (undefined - may 2013)
Edit: Polygon have cited the thread; neogaf should get press badges for e3 back :P