From my experience, weekly is important for a couple of reasons:

1. That's the cadence that most people's life runs on. It's easier to schedule and remember something that happens every Wednesday, than every other Wednesday.

2. You actually start to feel like friends with people fairly quickly when you see them weekly. It feels like a good balance between space and closeness for strangers to start becoming friends.

Starting a meetup is hard. If there's something relatively popular or trendy with the demographic of people you're hoping to make friends with, that might be something that will attract more people to your meetup quickly (AI, board games, investing, etc).

Even if you only get 1-2 great people to become regulars, you can cancel the meetup and still meet with them every week.