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For that reason I would never go b2c; in a bootstrapped b2b, if you make 10m-50m/y and some unicorn comes, you can still exist for a long time and this unicorn can buy you even. In an ad based b2c, there is no way.

There are a lot of b2c business models aside from ads (e.g. actually selling/charging money from customers)

Sure, but the example here was social media sites with millions of visitors and no monetisation plan besides ‘being the biggest’ & ‘something with ads … later’.

I'd love to know what their marketing spend was/is like. because it seems that they were purely word of mouth.

Once you have users, it can be very hard for a competitor to dislodge them. There used to be a ton of social media startups, and large company efforts like Google+. They mostly just flopped.