As has been pointed out many times, the "recently browsing" feature does not accurately list how many people are online. It only lists people who have recently refreshed the main forum page. People who are active, but reading a specific thread, are not listed.
For example, if you go to the main forum page, look down the right side of the page and see who has posted most recently in the top threads, within the last 5 minutes. Most of those people won't be listed at the bottom of the main forum page as "recently browsing". But, they're obviously here and active. Open any of the popular threads and scroll to the bottom and see who's recently been in the thread, and often they won't have been listed on the main forum page either.
At once point, during the Mayweather/McGregor fight, there were about 50-75 posters in that thread. The list on the main forum page, however, only had 20 people on it.
Like many of the "features" in this "upgrade", it is broken and doesn't work like it's supposed to.