two things...

all exchanges are essentially "meetup" groups, that is how the first exchanges/bourses/markets started. Whether people "meetup" online, or in person, you are joining a group for the purpose of transacting. Also, I think that you are mistaking differences in scale to differences in kind. A market is a market, whether it is a large market, or a small one. The goals and methods are the same. That is also why Satoshi Square is not the same as other meetup groups, people are coming prepared to trade (cash and bitcoins), because they know the goals of the meetup group they are attending. If I show up at a meetup group that is based on providing information, and ask to buy (or sell) $10,000 worth of bitcoin, I most likely won't be able to do that, the people who can facilitate transactions of that size may not be present, or may not be prepared. Like all meetup groups, clarity of focus and stated goals is what is important. There is no reason that you can't have multi-dimensional meetup groups, and that is actually how I see this idea spreading to multiple days of the week.