At its apex, Tumblr had more users than both Instagram, now estimated to be worth close to $200 billion to parent Facebook, and Pinterest, which has a market cap of nearly $18 billion. In 2013, Tumblr sold to Yahoo for $1.1 billion. On Monday, the parent company of WordPress.com bought it for a pittance.

The precise amount is hard to pin down but insiders have observed that there are modest homes in Silicon Valley that might be comparable in price. Marissa Mayer, Yahoo’s former chief executive, once described Tumblr as an...