Tumblr

Tumblr was a social networking website on Earth in the 2010s. Courtney Woods posted pictures of her adventure with the Twelfth Doctor and Clara Oswald on the Moon in 2049 on her Tumblr. Lundvik, a native of 2049, said her granny used to put things on Tumblr. The Doctor was angry to learn of pictures of himself being put online, but was able to use the Tumblr posting to establish contact with Courtney's phone. (TV: Kill the Moon)

In 2015, many of the selfies taken at San Diego Comic-Con were posted onto the con's Tumblr. (COMIC: Selfie)

When the Eleventh Doctor told Alice Obiefune that she could not behead future President Thomas Jefferson, Alice responded that she was not planning to do so, but that he probably deserved it and that Tumblr would vindicate her, if not history. (COMIC: The Long Con)

Behind the scenes
After search-wise.net and eBay, Tumblr is the third website portrayed in televised Doctor Who Universe media which actually exists on the Internet, and was not put online as a marketing tie-in by the British Broadcasting Corporation. It is the first real-world website which is portrayed as both looking and working as it does in reality.