With the most recent version of the app installed, just head into the Uber feed, tap "contact," and then hit "chat." This way, you can send your driver useful info -- like your exact location -- without ever having to leave the app. Or, you could just troll them with emojis, it's entirely up to you. And, of course, it's great for those of you who aren't fond of having one-to-one conversations during the ride.
On the other end, drivers will have the messages read aloud to them, so as not to distract them from the road. They can then respond with a simple thumbs up. Like most chat apps, Uber will tell you when your message is delivered, and read. It's still unclear whether you'll be able to carry on messaging after your ride ends. But, that would cause way too many privacy hurdles for it to work.