There's a limit to how many people you can sustain social relationships with. After all, to do so, you have to spend time with each. Intimate relationships take a lot of time; more distant ones less so, but nonetheless, you can't keep up with -everybody-.
Research from the 1990s suggest that 150 people is about the most you can sustain relationships with.
Mobile and online game developers tend to think that the ideal is "one big world," in which everyone can encounter and compete with everyone else.
Dunbar's number suggests otherwise. Encountering and competing with someone from the other side of the globe whom you are never likely to encounter again is... otiose. Encountering and competing with someone you recognize, and have competed against before, is more emotionally impactful, and meaningful.
HEROES CHARGE has shards of about 500 people; everyone on the shard joins the game within a restricted period of time. You are competing against players who joined around the time you did, not against long-term players of vastly more power. And if you play well, and (of course) monetize at some point, you can remain competitive at a high level, even if whales out-pay you.
Social connection is important; players are much more likely to remain in a game if they develop social connections to other players, whether those connections are supportive (e.g., in a guild) or rivalrous. Knowing both your friends and your opponents supports your desire to play.
So "one big world" may be a technologically ambitious goal to pursue; but it's actually a bad thing, from a player retention perspective.
Let players get to know each other -- both as friends, and as rivals, and possibly as frenemies.
Shard your game.