>The differences in CPC between an affluent white male working in tech in San Francisco and a poor black woman selling peanuts on the street in Rufisque, Senegal (which isn't even the most extreme opposite one could think of) is proof of that.
Why then are advertising giants like FB so eagerly expanding into "3rd world" markets and offering "free internet"?
The French government.
As for expanding into "3rd world" markets, the average monthly Senegalese salary appears to be 139€. And this is a country considered to be doing well in the region.
They're expanding and offering "free internet" because there's nowhere else to go. Because even though it's poor now, there's growth, and they want to be established and deeply encroached when those markets will become worth it. Because they believe better and wider internet coverage will spur even more growth.
The only reason these companies can exist and be worth anything today is because of all the groundwork and investment that has been and continuously is made.
Where else could they expand?
I'd say it's pretty obviously a calculated risk that they'll develop in value once hooked. Didn't Facebook have a scheme that gave net access in the developing world that re-created the AOL model? Free internet, but it's only FB internet.
The complete or comparative lack of regulation in many areas probably encourages too.
Worked for tobacco companies, works for most other industries.
They need a sufficient internet infrastructure. People with access to it.
People with money to spend.
People with enough peace of mind to spend it on something else than absolute essentials.
People with enough peace of mind and free time to leisurely waste on mostly useless websites.
People to whom the physical goods of a great number of their advertisers can be delivered (infrastructure & post system).
Eyeballs alone are worthless.
The differences in CPC between an affluent white male working in tech in San Francisco and a poor black woman selling peanuts on the street in Rufisque, Senegal (which isn't even the most extreme opposite one could think of) is proof of that.