But, then, how is Facebook so profitable?

Are they lying?

No.

They are growing.

More and more people sign up to Facebook, and more and more businesses hear about how many people are on Facebook.

It seems like a huge opportunity.

TV shows and award-winning movies are made about Facebook.

...

Eventually, though, and this might take a long time, but it is finite, everyone will have tried Facebook ads and know that they are useless. Eventually, after 10 million businesses have invested $1000 each, and Facebook has earned $10 billion in revenue in total, then they will have run out of new customers and their revenue will dry up.

A useless product is never sustainable.

I wish I could short Facebook.