This isn’t really telling us much.

For instance, if we compare FB and Reddit.

Facebook is a social media platform that doesn’t offer anything other than connecting people and groups, or for business pages and events. It’s not useful for external searches to take you there because it tries to force you to logon or get the app every time to view anything, and the search function absolutely sucks. I’d imagine most searches for “facebook” are people putting the term in google search to reach the platform or searching google to see if a person or business is on facebook. The search term is going to decline because most everyone that uses FB is already on it and FB is mostly pointless to search.

Reddit is a collection of subs , some of wich contain desirable information. It can be tech help, mechanical help, memes, history information, etc. So people absolutely search reddit + (thing they are hoping to find).

Iow, it’s a measure of usefulness via search, not popularity or user numbers. The more restrictive to search, the less people will look for it.