Among all people sure, but among people in the dating demographic? I would not be surprised at all if more people in prime dating ages are on IG than FB (especially in an active manner, do young people post on FB in any real way?).
I think you’ve gone astray equating young with prime dating, all you need is like seeking like, and both using FB is probably a good start.
Facebook has pitched this showing young people so I assumed that's their market. I guess the boomer dating market is probably underserved, maybe that's what they're actually going for.
I agree for I what I assume you mean by prime dating ages, but think there are more prime dating ages which are not so "prime" in other ways, if you will.