I'll provide my two cents, in the case of group activities. It seems awfully convenient to have a single site (e.g., meetup) for, say, if I'm traveling to a new city and looking for something to do.

It'd seem like an unnecessary barrier to entry to have to find the locally hosted instance of every particular problem, every location I go.

Knowing I can use Yelp or Groupon etc to find local restaurants or deals anywhere is actually pretty helpful.

To get to the core of your premise:

> I thought there was no reason that signals needed to go through server farms in California just to organize a local dinner, have a local marketplace, classes, dating, or a host of other things.

You might think there is no reason for this; but users of these services aren't concerned with this problem. If your selling point is "Hey, don't you hate how this one big corporation has a monopoly on <marketplace> <group activities> etc?", you're going to hear crickets. This isn't a customer problem.