Blockchains generally replicate every bit of data to every node in the network for independent validation. If you have a lot of nodes run by different operators, it puts a limit on the amount of data you can replicate out and still have the whole network reach consensus in a timely manner. Networks that promise high throughput with low transaction latency typically have fewer nodes run by a small number of operators. At that point, whats the point? Just have it be in a big scale-out database where itll be faster and more reliable.
Generally speaking, social networking data doesnt need global consistency. Blockchain is just the wrong kind of database for it.