YY, this is what I am thinking.
I know that age 18-22, which is when most people go to university, is a time when things like depression and anxiety aren't that uncommon, and I believe (could be wrong) that it's also when some other mental health issues tend to start showing if you have them - and if you're badly depressed, sometimes it can be extremely hard to talk and easier to write.
Obviously there's a flipside in that you need to really work to make it feel like a network, a community where people know each other and that would be extremely difficult to do purely online. But you could try.
Re. bums on seats - when I was at university, if you were persistently absent without permission, you could be fined! Never happened to me (not for lack of absence, for lack of being a cocky git about skiving, which tended to be the real reason they'd bother to fine you). However, my lectures weren't mandatory. Where I am now, I take attendance and they are required to attend. If they persistently truant, they can get chucked out. I'm not sure what I think of it.
Something I find very difficult is that I am supposed to mark them on participation in seminars. Obviously that's important and it's not fair not to contribute. But with some of them, it clearly is shyness. So I tend to put them in groups, then eavesdrop the quiet ones, and then I'll say something like 'right, Jane, you had a brilliant point about x just now'. But I suspect some of the Janes (it is usually women) would be quite able to write down what they thought if need be.
Sorry, I see that all of this is getting a long way away from the original question and I hope the OP isn't offended. 