Except you would still be more likely to target other parents in the same close-by neighborhood, which would lead to a lot of repeat "business", at which point you would develop relationships with your neighbors with kids of a similar age.

I mean, a business accidentally creating closer, friendlier communities is great, but I think it'd still be a financial failure in the end, because once you've met all your neighbors with kids and trust them, you can just friend them on Facebook and say "Hey, can anyone watch Timmy while I run to the grocery?"

Right, but the OP's point is that if I watch your kid for you, and you think I'm doing an OK job, then you and I can reach an arrangement separate from UberSitting. Say UberSitting charges $15/hr, where I get $10/hr and they get $5/hr. When you pick up your kid, I could say "hey, if you pay me $12/hr, then you'll save $3/hr" and then I make an extra $2/hr and UberSitting makes nothing. In other words, once you make that initial connection, you don't need UberSitting anymore.

This is one of the reasons why those cleaning service companies like HomeJoy have floundered, btw.

The on-the-fly nature of Uber is convenient, if you wanted someone to watch your kids NOW, your pre existing relationships might not work out.