They're already free to do this in theory with social network sites that don't require logins to see people's data (i.e. Twitter clones), but hopefully the EU's GDPR would make it easy to bring a case against Facebook if they were using the ingested data for any purpose other than supporting their own users.
The competing social network would probably need to have a privacy policy saying that other sites can ingest their data only if the remote user's "friend request" is accepted by the local user, and the data can only be ingested for a set of prescribed purposes.
The competing social network would probably need to have a privacy policy saying that other sites can ingest their data only if the remote user's "friend request" is accepted by the local user, and the data can only be ingested for a set of prescribed purposes.