PS Plus is a game subscription. It isn't my impression, it's a fact.
Even Ampere mention its themselves in
their article where they speculate about Spartacus rumors:
A game subscription means that you pay a subscription and you get some pay to play. This is what all the game subscripition have in common, the ones included in the graph and the ones missing there like PS Plus.
Obviously each subscription is different: some are download only, some are cloud only, some offer both, each has different extra perks in addition to games (online multiplayer access, discounts, exclusive dlcs, demos...), many of them differ on the platforms supported, etc. There are many differences between all of them. But the common denominator is that you pay a subscription and you get access to a collection of games (plus depending on the case additional stuff).
No, MS wasn't over 50M Gold in early 360 days.
MS announced
48M Xbox Live MAU in 2016, their record back then. Notice that it isn't Gold subs, which are a portion of these XBL MAU since players can use XBL on multiple consoles, PC or mobile without paying Gold. In the same way that the 47M Plus subs are a portion of the PSN 111MAU and PSN can be used in multiple consoles and the phone app. And well, Gold subs can't be bigger than XBL MAU.
4 years before discontinuing Xbox 360 (and 7 after its release),
Xbox announced 50M XBL downloads, meaning Xbox Live users downloaded 50M items that could be game demos, movie trailers, Xbox Live Arcade titles, dlcs or music videos.