[edanm]

Gmail could probably cost money and have a big userbase (at least now, when so many people know about it and use it).

But I think a lot of the community sites can't charge money. I don't see Twitter, Reddit, StackOverflow and other such community sites gaining nearly enough users that way.

[skorgu]

Stack Overflow is explicitly a rebellion against Expertsexchange which has a free option with lots of upsell.

[ahoyhere]

I'm pretty sure that there's at least 100,000+ people who would gladly pay for premium Twitter services. The amount people pay for superior Twitter clients and other ecosystems is an example.

Granted the network effect is still in effect, but it could be a killer freemium.