There are, to a first approximation, hardly any terrorists in the West, compared to all the other usual causes of premature death (car accidents, suicide, and in the US police shootings & gun accidents).

Of the organized terrorists, choice of target isn't going to be influenced by an app; it will be driven by symbolism (Twin Towers), political significance (Charlie Hebdo), availability, and the general fact that large areas of big cities are full of people (7/7 London bombings). There's no complex optimization going on here. Bataclan also appears to have been a vendetta: https://www.ft.com/content/d322a2d8-8df6-11e5-8be4-3506bf20c...

Of the unorganized terrorists, the mass shooters: these seem to be extensions of personal grudges where the person goes to their school or workplace and opens fire. Even when the target is a nightclub (Orlando), the choice of target is political (in that case, driven by homophobia).

I'm not sure there are many terrorists anywhere else, at least by the normal definition. How many people in Iraq have been killed by civilians with car bombs versus all other causes of death? The point of terrorism isn't to be a likely killer, but to cause irrational terror, thus the name.

Yeah, because terrorists need help knowing when JFK, LAX or LHR airports are busy.