Let's suppose that one day Synereo provide exactly the same as facebook exept that users get rewarded by using the social network
I already documented the economics of that. Ad revenue would never pay enough to motivate users to share. They share for much more valuable reasons. No one is going to work for $1 to $10 per day. Ads in developing countries pay less, because people spend less.
(without speaking about huge privacy issues in centralised social networks), isn't that enough for people to switch to Synereo ?
No. That is what the past experiments of many decentralized attempts have shown.
Moreover, the russian social network Odnoklassniki got destroyed by facebook and Vkontakte which did not provide added value, which annile your point of needing a niche.
Facebook won because it had more users or Odnoklassniki won because of Russian language niche and Russian culture focus. This proves the point of why you can't beat Facebook at its own game. You have to have an untapped demanded niche.