GNU Social isn't P2P, but federated. You don't have to host your own node to be able to use it, which massively raises the barrier to entry, but instead lets you register on any server in the federation as an entry point into the network.
There are both upsides and downsides, of course. It's more centralized along with having censorship by your hosting node. However, it doesn't force you to leak your IP address to attackers or run software and be a sysadmin, along with saving bandwidth. It also doesn't have some of the difficulties that proper P2P does: Twister is on top of Bitcoin (costs to use) while Sone is over Freenet (Sybil attacks and routing issues)
There are both upsides and downsides, of course. It's more centralized along with having censorship by your hosting node. However, it doesn't force you to leak your IP address to attackers or run software and be a sysadmin, along with saving bandwidth. It also doesn't have some of the difficulties that proper P2P does: Twister is on top of Bitcoin (costs to use) while Sone is over Freenet (Sybil attacks and routing issues)