If I recall, it was optional for CounterStrike1.4, but then required for CS1.6.
The PC bunch were crying that they'd never use this DRM'd buggy POS.
That must have been 16+ years ago. Steam has come a long way since then, and has been credited [0] with helping reduce PC game piracy over the years. Likely because steam just works for the most part, and is as easy if not easier than the alternative.
I just don't see how Facebook can really disrupt this given Steams head start and positive mind share (not to mention the libraries of games that users have likely accrued over the years)
0: http://www.digital-digest.com/news-62904-PC-Gaming-Piracy-On...