Compared to Twitter/YouTube/Spotify/Facebook, Meetup is a pretty simple app. Most activity is localized to a single group, it's 100% user generated content, and it's pure digital. "230,000 organizers who create an average of 15,000 in-person events per day" isn't in the same ballpark as those other services.

Maybe they have Hard Tech problems like dealing with griefers, but the basic group system looks pretty straightforward to scale with something like Google App Engine.

That said, I'm sure most of Meetup's employees are not engineering.