Transportation is part of Messenger and can be used directly from conversations. Users can click on an address and in conversation and in the menu that appears click on “request a ride”. Alternatively, they can click on the auto-emoji.
It seems intended that in the future, users can select from multiple transport providers, but as yet there is only Uber integration. Of that service, people who have applied for a ride even get status updates about the time it will take for their transportation.
Facebook test the functionality with a limited group of users in the US, but says that other countries will follow soon.