It isn't quite yet in the millions:

Within the span of two days, more than 700,000 new users have joined Xiaohongshu, Reuters reported, citing a person close to the company. CNN has reached out to Xiaohongshu for comment.

https://hexbear.net/comment/5821835

Allowing for a day since the article was published and for Americans already set up on the platform, it's probably around a mil.