If you have a hard time imagining Twitter collapsing or Facebook becoming irrelevant, remember that there was a time that Yahoo! was synonymous with The Internet and it seemed like nothing could stop it from world domination. It had purchased Flickr which was an absolute thousand-pound-giant-gorilla destination that OWNED online images. It had all the cards to play to the end.
These things never last. Money isn't smart and it always destroys the thing if left to its own devices.
