Question: How did the select the 300 Reddit communities for comparison?
Answer: They selected all the subreddits from January 2013 to December 2014 with at least 500 words in the vocabulary and at least 4 months of the subreddit's history. They also removed communities with the bulk of the contributions are in foreign language.
Alternative answer: They collect subreddits from January 2013 to December 2014,2 for which there are at
least 500 words in the vocabulary used to estimate the measures,
in at least 4 months of the subreddit’s history. They compute our measures over the comments written by users in a community in time windows of months, for each sufficiently active month, and manually remove communities where the bulk of the contributions are in a foreign language.