He has two assumptions:
- You can make a profit in the industry of your advertisers. This is a huge if, and is impacted by scale, expertise, management focus, time/maturity, branding and many other factors.
- The return on investment you can make in industry X is higher than the return on investment you can make by expanding your operations in your current industry. This could be true for social sites, where increased investment (i.e. programmer time) might not get them many more users, but they still have to compare the possible revenue to the increased profit from firing some programmers and going into maintenance mode.
These to are the main reasons why you see companies outsourcing non-core activities (like cleaning and even product development), the markup they now have to pay on the service is lower than their internal cost of capital.