The xx announce tour dates by mailing free tickets to fans

What if you are invited to your favorite band’s concert by your favorite band?
Now it has happened to the xx’s fans!
Recently, the xx are back with a new song “On Hold” and a new album “I See You”; and after some mysterious signals, fans began receiving invitations and free tickets to a series of then-unannounced tour dates.
At the beginning there were some Spotify streams and other suspicious moves on Shazam.
At the end of it all, it culminated to the great social media sharing campaign by fans them-selves. In fact, fans shared on Instagram and Twitter tickets that were mailed to them from the band inviting them and a friend to see them on tour.
Fans received letters in the mail from the band, informing them that they have been given free tickets to previously unannounced concerts.
The xx's announce was something like that “We would like to invite you and a friend to our show.”
There's a very important lesson to be learned from all this: it always pays to tick the box “Yes, send me information” when you sign up to a group, mailing list o to pre-order an album.