Has anyone

[ZiNgA BuRgA]

I think you're misunderstanding things on a number of levels.
I strongly doubt these big sites rely solely on donations - they probably are funded primarily through advertising or affiliation or similar.  I've seen a number of forums do this.  Donations just provide extra income to the forum owners (after all, users don't know how much the owner is really earning through ads).
Generally, subtle advertising can more than easily cover for hosting costs.  I'm on a dedicated server here, and not the cheapest one ($66/mo).  My site doesn't even need this much power - it could easily do fine on a $10/mo VPS server with 1GB RAM, but ad revenue easily covers for this server (and I'm too lazy to switch).  Any forum which gets a decent amount of traffic, and has advertising, I will seriously doubt relies on donations to keep afloat.  And even if it does, the owner must really consider the stability of such an income stream - typically these can vary a lot, and if you have to match payments with earnings in this situation, you're either going to have to leave a big margin of error, or be willing to contribute funds yourself (or let the site go down, which is unlikely for a large forum owner).
As such, a subscription payment is hardly anything compared to the hosting they have to manage.

Of course, the above is only for established forums, which probably won't convert to another system, so it's rather a moot point.


I think the rest of your post is a bit of an overstatement...