Yesterday two airliners were escorted down to Atlanta USA due to bomb threats. There was also one earlier in the week over New York. Not long ago the Spanish rail network was targeted and late last year the RAF escorted a plane down to Stanstead. There will have been other cases we have not heard about.
The beauty of hoaxes, from the perpetrator's point of view, is that they are as cheap as a text message. All they have to be is credible. The number and type of targets is almost limitless and because of that they cost the victim a huge amount in resources, disruption and security. These things, and vigilance, have to be kept up for as long as the campaign goes on. The authorities could never ignore a hoax because the first time they did they would be hit by the real thing.