[Jtsummers]

This is a good point. They're driven by advertising revenue (primary revenue) and collecting market data (to better target advertising, making them a good option for buyers of advertising). Social networks, chat, flight data, search, video watching (youtube), aggregated shopping data, all allow them to deliver advertising or collect data on users.

Looking at the services that have been killed over the years or stagnated, they don't allow enough data collection or ad delivery to justify spending money by Google. Either because they simply weren't well geared towards it, or because they didn't get enough uptake in the market.