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).