I think the most compelling reason to not build a business around ads is that it has a poor ROI. Just do the math, and even with the most optimistic estimations, you'll still be disappointed.
1 million uniques a month => 10,000,000 PV/month => $5 CPM => $500,000/month => $50,000/month => $600K/year.
Sounds great, but then you realize coming up with 1 million uniques by yourself is highly unlikely, and will probably depend on high costs as well.
Side Rant: Those people saying 10 million users is the next 1 million users... most of them are investors who invested in Twitter or Instagram. They have NO clue how hard it is to get even 100K users, let alone 10 million or 1 million. Give me a break.
1 million uniques a month => 10,000,000 PV/month => $5 CPM => $500,000/month => $50,000/month => $600K/year.
Sounds great, but then you realize coming up with 1 million uniques by yourself is highly unlikely, and will probably depend on high costs as well.
Side Rant: Those people saying 10 million users is the next 1 million users... most of them are investors who invested in Twitter or Instagram. They have NO clue how hard it is to get even 100K users, let alone 10 million or 1 million. Give me a break.