For some context to pram's comment, by essentially worthless he's referring to how Yahoo recently completely wrote off Tumblr's goodwill value from when they acquired it, which was most of the purchase price. In other words, Yahoo paid a premium because they thought Tumblr had a lot of potential, then... wrote it off.

https://en.wikipedia.org/wiki/Goodwill_(accounting)

For more context see:

2013 talk of 75% value being goodwill: http://money.cnn.com/2013/08/09/technology/yahoo-tumblr-good...

Then 2016 writedown: http://gizmodo.com/yahoos-latest-earnings-report-basically-c...