YouTube

YouTube was a website on the Internet on Earth in the 21st century.

After a wave of Shakri cubes arrived on Earth, people posted about them on YouTube. (TV: The Power of Three)

Behind the scenes
In the real world, the Twelfth Doctor title sequence was created by a fan of the show, Billy Hanshaw. Steven Moffat was impressed with the conceptual opening sequence that was popular on YouTube; liking how original it was, he got in contact with Hanshaw for creating an official title sequence.