if I had to explain the difference between FB/birbsite and masto

(IMHO!) it's like the difference between trying to find a table to sit at lunch vs going to a club before/after school

FB/birbsite relies on you already knowing people and assuming that you talk to each other on a regular basis

& while masto might seem smaller, but with all the different instances there's a certain common ground that helps you make new friends via shared interests and learn new things from each other