Xt3.com is a social network for Catholics in Sydney, and beyond, although it’s unclear (to me) if they will keep the same URL for future World Youth Days. Maybe they are waiting to see how well it grows on its own before reaching out worldwide.
From Zenit.org:
It is hoped that Xt3 users will continue to connect after World Youth Day in July, to build upon friendships made and continue dialogue about what it means to be a young person of faith.
Xt3.com moderators are background checked to “ensure that Xt3 is a safe place for all involved“. Good moderation might get prohibitively expensive, and could slow down interactions, but it’s worth trying!