booking ride for someone else

Yes, you can. Uber & Lyft both acknowledge and encourage

Yes, you can!  You can simply enter or pin-drop a location where your friend needs a ride.

Many used to frown on this practice as it creates confusion, introduces anonymity among the passengers, and some abused it.  With that said, it seems to be a lot more accepted now, and both Uber and Lyft now encourage this.  

Uber's Response:
https://help.uber.com/h/454187f4-9b4e-4f9e-8dac-60b04910e8bc

Lyft's Response:
https://help.lyft.com/hc/en-us/articles/214583147-Policies-for-Passenger-Uses-of-Lyft#requesting_for_friends

Tips:

- When you call a ride for someone else, send a screenshot of the car details.

- Make sure the ride has ended when intended.  The driver *could* not close out the ride after drop-off, and ride around to make more money.   I am not saying they would, but you do not want an open transaction out there.

- Make sure to contact your driver.  Uber now communicates the "caller's" location per GPS to the drivers.  Sometimes drivers will directly go to that location rather than the specified.

- Remember that from your one account, you can only call one car at a time, and you must wait until the ride ends to call another.  Some people have faced issue.  Think of a scenario where you step out of a bar with a friend, you kindly call him a ride, and you try to call yoruself one...but wait!  No!  you can't until your friend gets all the way home.

UPDATE June, 2017:

ippei 1202

Uber Driver, CEO of TaxiFareFinder and Unleashed, LLC