Great questions. I couldn't stop thinking about #3 while watching that video. Edit -- Seems like he answered #1 in another commment: Right. It turns out a lot

Great questions. I couldn't stop thinking about #3 while watching that video. Edit -- Seems like he answered #1 in another commment: Right. It turns out a lot of the work comes from gluing the various services together. For ex, when a user signs up with an email account, and then forgets and signs in with Facebook next time. This takes care of deduping accounts, and flattening the social data, etc.