Meetup.com is useful because they have a very effective system of spreading the word (through their mailings). Besides, the Meetup system is good for organizing and scheduling and in certain areas (like software technologies or entrepreneurship) it is used by many people almost daily. If you are interested in increasing the number of people attending I think it is worth trying.

Is there a specific reason why LessWrong meetups are not being published in meetup.com?