BWAHAHHAHHAHHA. Ha. heh. whee! It's a good product but good luck on that part, Joel. Support costs for massive free products are probably more expensive than you imagine.
Joel is heavily involved in StackOverflow.com and the whole StackExchange universe, so it's likely he has a good handle on support costs for hosted products. Fog Creek also offers a hosted version of FogBugz (free, I think, to students and open source developers) so that's more experience in support costs and issues.
Facebook is expensive to operate but I am willing to bet that that is because they host so many pictures (try to dig up the actually numbers -- it is crazy).
Joel will only host a relatively small amount of text -- which should be a lot less expensive.
"Support costs" refers to the costs of supporting users, not the cost of hosting content. This includes the cost of someone replying to the presumably tens of thousands of emails requesting support and trawling through it harvesting bug reports, feature requests, giving users fixes, telling users politely to press the right button if they want it to work etc.
BWAHAHHAHHAHHA. Ha. heh. whee! It's a good product but good luck on that part, Joel. Support costs for massive free products are probably more expensive than you imagine.