Why? They're tracking to a likely $18+ billion in sales in the next four quarters. That's obviously an extremely substantial business.
By comparison SAP is Europe's largest tech company and does $21 billion in sales. Facebook will surpass them within eight quarters (at a mere ~14 years old as a company).
It's just the common line that you see when people talk about revenue possibilities and Facebook. There's a tendency to talk about Facebook's potential (rather than their current state) because they're a company with a lot of it.
By comparison SAP is Europe's largest tech company and does $21 billion in sales. Facebook will surpass them within eight quarters (at a mere ~14 years old as a company).