Finding a babysitter for Saturday night can be tough in neighborhoods where people rarely know one another's names. But that's changing, thanks to Nextdoor, a local social network designed to connect neighbors.Over the past year or so, more than 12,600 neighborhoods in 50 states have joined Nextdoor, and about 1 million messages are posted every day. San Francisco, Seattle and Denver neighborhoods have each surpassed 90 percent participation, but Nextdoor isn't just for urban areas. It's for everyone.Nextdoor members have been largely reliant on computers for access to the network, but they now can use a free app for iPhone and iPad. I spoke with Nirav Tolia, Nextdoor's co-founder, to find out the differences between the website and the app. Mostly, it's one of convenience.