This graph, created by user Wretchedlout on the Data Is Beautiful subreddit, is fascinating. It charts the growth of Steam's concurrent users and the games they tend to play and does a great job of highlighting a number of interesting trends.
(Pro Tip: click on the image, some of the words are difficult to read.)
The most obvious thing to note is that PC gaming, on Steam, has seen consistent, dramatic growth — but things really amped up around 2009 and hasn't stopped since. More and more people are using Steam and I don't see that trend stopping any time soon.
The interesting thing for me is the path of the games themselves. Dota 2 is just ridiculous. It's the most played game on Steam by miles. And what we're really seeing is this: the recent growth of Steam correlates pretty heavily with the growth of Dota 2. In a sense — Steam's rise is directly tied the popularity of Dota 2.
The popularity of Counter-Strike: Global Offensive isn't really surprising. What is surprising is the path it has taken. Simply put: it was a slow burn. Counter-Strike is a massive 'brand' and you'd expect it to perform well straight out the gate, but that's not how it played out. Looks like it took a while for long-time players to transition to Global Offensive, but now it's riding the crest of a popularity wave both among casual players and folks on the competitive scene. Everyone is happy with Global Offensive and it shows.
A final point of interest: games that aren't competitive, multiplayer focused titles. They have a completely different pattern. They kick off huge and slowly tail off. Sometimes the drop off is dramatic. Grand Theft Auto V is the best example of that. Huge spike upon release, but only a dedicated core of players remain.
It's super interesting to look at. You can check out more graphs here.