There is some sort of automated checking in going on; I haven't checked in even once in Swarm, and I've been asked by my friends if I'm a fan of this change because apparently, I've been checking in all over the city.
Your location/neighborhood is being updated automatically in the background, but you are not automatically checked into venues.
Considering how popular it is to inflate ego metrics, safe to assume 90% means 90% of people who use the app everyday, or something incredibly obtuse.
Not even Candy Crush can say 90% of users download a new game, etc. Let alone Foursquare. yawns
You/Foursquare really should clarify exactly what you mean by this - we keep hearing how "successful" Swarm is but it seems most [former] users (including me) and their friends (including mine) stopped using it when (or shortly after) the switch happened.
Right now Foursquare sounds like Google in the early days of Google+ where they were touting how much people were posting but everybody was saying they never saw any activity in their stream.