I heard on launch that it would let you join friends via the in game friends list on any platform that was patched out but a reddit user has noticed the potentially the same happening again in the new Battle Royal PvP game mode.
https://www.reddit.com/r/FORTnITE/comments/70fj38/fortnite_just_made_console_history_proof_of_cross/
Edit
was just playing Fortnite on my Playstation 4 and ran into a group of teamers at the end of Battleroyale... surprise... and noticed I got killed by an odd gamertag.
zCypher Nine
https://www.twitch.tv/videos/175127882 here is a link of the twitch VOD as proof of whom I was killed by, his friends and the fact I was using a PS4.
That gamer tag is IMPOSSIBLE to create on PS4 because of the space. On PSN you are only able to add underscores into gamertags. So I typed it in with an underscore just figuring it was a glitch. Nope, no dice.
I am lucky enough to own both a PS4 and XBOX 1. So I switch over to my XB1 and boom, instant results. I also searched his friend he was teaming with RatoDoGlobal... boom instant results.
https://imgur.com/a/f17M9 screenshots of proof they are real gamer tags on xbox and not ps4
This is HUGE. Sony and Microsoft do not allow crossplatform play. To my knowledge this has never happened before. WTF just happened? And fuck you for the first documented case of this ever happening being a group of dudes cheating.
https://www.twitch.tv/videos/175120924 full clip of the entire match start to finish, incase anyone cares to dig into any other gamertags for further evidence of crossplay
https://www.reddit.com/r/FORTnITE/comments/70fj38/fortnite_just_made_console_history_proof_of_cross/
Edit
The verge have picked it up
https://www.theverge.com/2017/9/18/16327964/xbox-playstation-4-cross-platform-play-fortnite
Epic have confirmed it as a glitch that has now been corrected