What's interesting is that Slack (the company) was founded in 2009 and yet the initial release of Slack was in 2013. That's 4 years of development, that seems like quite a long time.
Flickr: They first built a game, it didn't do well, so they pivoted to focus on the image hosting they built for the game.
Games are hard.
I was sure there were more examples, though.
Games are hard.
This is almost the exact thing that happened with Butterfield's first venture, Flickr, which was built on a web platform they wrote for a multiplayer online game called Game Neverending (which is why Flickr URLs have/had "gne" in their URLs).