I wouldn't belong to a site that tried to force me to donate. I think that donating should be done when you want it. Donations are not payments so I don't think that a site that wants to market itself as "free" should push users too hard to donate.

Personally, I don't always have money to donate. I don't donate as much as I would like to to Mene at least. I agree with the fact that if a site is so worried about money that it should make the site a pay site instead of a free one.