If you take a look at https://pushshift.io/, comment/post activity has seen significant growth in 2018. Of course, this is a completely different metric from unique visitors. I mod a large-ish (top 20 by subscriber count) sub so I can see traffic stats for the sub. Our monthly unique visitor count barely grew over 2018 although that is not very representative of site activity as a whole either.

The team at Reddit has all the data. They will know how to leverage/present them in order to raise more money. Web ranking is not the only thing used in valuation.