Probably even fewer than it appears online, since many forum members belong to more than one trad forum using different screen-names.
Yes, you have to filter out the duplicate names that refer to the same person, and THEN multiply by 25 (or possibly more).
Another point -- just because the young people at your chapel have cell phones and are "online" doesn't mean they ever visit Trad Catholic forums. They could still be "off the radar" so to speak.
How many large families keep their children off the Net for the most part, even up to age 18? And so on.
And when a young lady turns 18 and is finally "allowed to join Catholic forums", is she instantly going to have the habit and inclination? Is she going to run out on her 18th birthday and sign up for every Catholic forum that exists, like a starving person at an all-you-can-eat buffet? On the contrary, she might simply adopt her parents' aversion to posting online, just for the sake of "being a good girl" or embracing the
spirit of the law, as it were. Just because it's such a small thing to her.
She will have been forced to find other things to do with her time, and those habits will still be with her. She won't instantly find the need to post online, nor will she instantly break herself away from all the other activities she used to spend her days doing.