This is a good point. A search engine like duckduckgo can choose to make much less by not tracking so much user data...and its still "enough". There are other vectors to bootstrap a friends/family sharing system that values privacy. Such a system can make a lot less than FB and still be "enough".
That would depend on how the costs of running servers scale with the number of users. If they scale faster, duck duck go would need to increase its income somehow.
