Back of the envelope calculations are fun. I think a typical app will make $1-$10 (ad based vs. virtual currency based) per month for each 1000 monthly active users. Let's say about 20% of apps are virtual currency based, so $2.8. The top 20 apps on facebook have total 179M active users, so they would make $6M per year. If the top 20 accounts for 50% of traffic, that would pin total Facebook app earnings to $12M/year.<p>If I recall correctly, Joel has once said that the cost of office space accounts for something like 6% of their revenue. At another time I think he mentioned their new office costs $50000/month, so their revenue would be $10M/year.<p>So Fog Creek software makes as much as all apps on Facebook combined?-)<p>Edit: Read the article. Even Zynga alone seems to be making more than my whole estimate. Oopsie.