TERA Online has been doing very well since it debuted on Steam this month, because according to the publisher, it's now the most played MMO on the platform. It's leading the race of "all-time peak concurrency on Steam," right ahead of Neverwinter, Rift, Archeage, and Elder Scrolls Online. Ouch to that last one.
Even though I got tired of the grind after a while, I enjoyed TERA even back at launch as a paid MMO because of the fun combat system, so it's great to see it thriving on Steam. Check out the infographic below for more info.