I am not sure when a meetup becomes an institution, but a 10-year-old monthly gathering of tech heads in New York City called NY Tech Meetup has definitely crossed the bar. It has more than 40,000 members and claims to be the largest tech meetup group in the world.
Every session I have attended has had more attendees than the previous one, although the format remains the same; three batches of three presenters describing their breakout tech inventions and then answering questions from the audience, plus one bonus presentation to mix things up.