Sony have recently announced the Playstation 4 has sold though to consumers 35.9 million units. 5.7m units were sold during the "holiday season", (22nd November through 2nd January).
Whereas Playstation Plus memberships has increased by 60% compared to 2014. Most likely owing due to the need to pay for online multiplayer.
It really is staggering at how much of a behemoth the PS4 has been. With 2016 having a host of games coming out for it, the 50 million mark is coming sooner rather than later.
EDIT: just noticed at how big picture is lulz.