After raising $300 million, Reddit looks like a growing force in the world of social media. The company is valued at $3 billion, as of the latest funding round. That’s up from its prior $1.8 billion valuation in July 2017, according to data from PitchBook.

The 67% growth is notable given that shares of Facebook are essentially flat over the same period. Facebook shares traded at $169 in July 2017. Today they’re $166.

With...