I have been co-organizing the SlateStarCodex online meetup series with ~40-140 participants in each meetup. We've experimented to get it to work smoothly and have some tips.

This advice is for meetups which have a talk and QA and post-talk socializing. I will appreciate similar tips from meetup organizers.

For   socializing, we use  breakout rooms as mentioned below; but for pure socializing,  Icebreaker works even better, putting people together for short conversations (like "speed dating"). This worked well in some Effective Altruism Icebreaker sessions. 

Another good format is Rump Sessions: Four-minute Lightning talks, given by on-the-spot volunteers (no need to pre-apply); and for each,  the audience votes for a  three-minute extension if the speaker wants. That worked well for the preliminary session in the series, but all the rest had invited speakers.

Here's how we do it.

• We  advertise using a regular (non-event) post on  LessWrong, as LessWrong does not support non-geographical events. We also make  an event post under LessWrong Tel Aviv,  allowing people to get a notification or see this in the event page.
• We also post to /r/slatestarcodex
• We post to Facebook. Few Facebook groups in the rationality-sphere are really active, so we do not share to any of those. (I do post to my local LessWrong Israel group.)
• We have a MailChimp mailing list, which has worked very well to get out the word. People can sign up when they register for a meetup.
• For each session, we ask people to register; then we send them an invitation. So, we do not publicize a direct video-meet link. This approach gives us partial visibility into who will join.
• We use Google Meet because the paid version allows more than 100 participants, and the paid version of Zoom does not. Usually someone can offer a paid version from their work or school, but note that the owner of the account will need to administer the call, so the owner cannot just hand off to the meeting organizers.
• I suggest in the invitation and in my introduction that --  both for the main event and the after-talk socializing -- participants turn on  video, and connect with their real name, for social connectivity. However, this is not mandatory.
• We do it Sunday 18:30 UTC because this accommodates the US, Europe, and Israel times of day and workdays.  (This leaves out areas east of Israel to the Pacific, unfortunately).
• Schedule: I connect 15 minutes before the session to test the audiovisuals. I recommend that the speaker connect a few minutes earlier, as we have had audiovisual problems in the past. The session  begins 1 minute after the scheduled time, and I give a 2-minute technical introduction about muting mics, videorooms, etc., and a 30-second intro for the speaker. The speaker gives a talk for 0 to 90 minutes, and then we do Q&A. After that, we go to videorooms.
• We ask everyone to mute mics to minimize background noise. I mute people if they forget to  do it themselves.
• Participants chat freely in text chat during the whole thing.
• Participants ask questions in text chat, prefixed by a "Q". My co-organizer curates them and feeds me questions to read out. This approach, compared to oral questions, shares the time fairly, avoiding  time-hogs. It also supports those with a bad audio connection. Afterwards, we do allow oral questions if there is time.
• For the last phase, following Q&A, we share links to 3-4 Jitsi videorooms for socializing. Participants can switch between rooms. This is meant to simulate post-talk chats in a face-to-face conference.