The argument that it wouldn't make sense for them to communicate to a handful of lunatics watching their every move through social media, doesn't really hold up. They don't have to be geniuses to know that U2 fans follow amongst others Morleigh's instagram. Just look at some of the comments and user names. Very subtle
And even if only, say, 50 fans are following Morleigh's instagram, it's plain obvious that whatever relevant post she makes will be spread through facebook, forums etc, ultimately reaching a much larger fan base. I'm not saying it was on purpose. I'm divided on that question (odd timing and snippet of a song vs U2's subpar marketing team). But in any case they are well aware that what they and everyone close to the band posts, will very quickly reach a wider fan base.