Social networks are valuable because of users. They gain value by attracting large numbers of users.
They have limited inherent value, and the majority have features which are easy enough for rivals to replicate.
But what matters beyond the network is how well it can be utilized by users. That layer of tools and functionality on top of the network is where Twitter could charge users. Particularly power users and businesses that can justify spending some money on it because they get so much value or even generate revenue from it.
It was not wildly successful.
Certainly not successful enough to replace advertising.
As for the network effect … that's the point of "the value comes from the users."