I run a meetup and most people find us by searching meetup.com or occasionally google.
I'm not 100% sure but his point my be: how do people find you once you quit Meetup.com? Cause attrition happens and at least Meetup provides a somewhat constant inflow of people.
Right but once you have a pool of 25-30 people, you're set ( for most activities).
For a while. Unless your pool of people is good about inviting new members (who also invite new members) that number only ever goes down, and eventually the decay grows exponentially.