I have no idea. According to that map on U2songs(?), a total of 30 letters were sent. That's too small an effort for any music corporation (what, are they going to have a person in the office randomly select 30 fan club members, in the US only, and send a letter to those fans, knowing in advance they are likely to post something guerilla marketing-style; what if none of those people were Twitter users? what if none of those fans thought to tell a fan site about their letters? noone would know! It's like the Willy Wonka Golden Ticket contest noone knows is happening). Same for the email...they really just sent it to 150 people? What would be the point of that for a world-class band like U2?
No one knows how many letters were sent. We just know that a few of the people who got one spoke up about it on social media. That doesn't mean hundreds of other people didn't get the letter. The band could have sent out 500 or 1,000 of them knowing that a large enough segment of the recipients would post about it on social media for it to spread. That's all they needed for it to have the desired effect.