We have a few of them already, you can take a look at those projects and learn from them.
Here's an example:
Sapien.
Every post, like, share you earn their native token, SPN.
It was launched I think around Q1 of 2019 but even with all the hype they still struggle to have a decent amount of users within their platform.
Another one is
Empowr. Almost entirely the same as Facebook but with tokens as a reward for each action you do within their platform.
Also struggling to have enough users to make it into the mainstream.
If you're planning on launching one; It should be unique, rewards its users, cannot be exploited for its rewards and has good tech support.
I Totally agreed with your opinion but I will digress a little. Those platforms might be struggling at present as Bitcoin was not popular for many years before it is gaining massive adoption. Anything on blockchain should just continue eventhough they are struggling at present, the future belong to them and as long they can implement new ideas. Nothing just get adopted one day. With Facebook Libra and Brave Browser, this will create a big awareness and people will be looking on how to get rewards when they do things online.