SITTERCITY- Babysitters aren't really the types of things you "share". Sure, if you've decided that, on second thought, you prefer Mary over Sarah, you might share Sarah's contact information with your friends- you know, so long as at the end of the day she isn't a sociopath.
As someone who started her family in an urban area, you know, the play-in-the-park by-day-but-don't-walk-through-the-park-by-night sort of urban area. It wasn't exactly the kind of place you bump into a high school neighbor to see if she wants to make a couple a bucks while the kids sleep.
I needed someone to watch our son for date nights but I also really needed someone so I could get a morning to myself just to go to the dentist, get groceries, etc. I met so many qualified people on SitterCity.com. Sure, there were some not-so-qualified sorts too.. "Your listing said you don't smoke...so do you just choose to bathe in Febreeze because you like the way it smells?"
As a new mom who didn't have a ton of opportunities to track down potential sitters, having a list of potential options at my finger tips was great. Seeing reviews helped guide me in the right direction. And with any in-home hire, the individuals references helped to seal the deal (ok, that, and the background check option).