I know that we are not Quora ;-), but due to the fact that we have the possibility to ping PO members everywhere and to write private messages, I sometimes feel tempted to make specific users aware of great questions they potentially would like and maybe even be able to answer.
For example, Arnold Neumaier might be interested in this question
http://www.physicsoverflow.org/20303/equivalence-moris-formalism-zwanzigs-heat-bath-procedure
Or Urs Schreiber and José Figueroa-O'Farrill might like and be able to say something helpful about this question
http://www.physicsoverflow.org/20309/exotic-spheres-in-string-m-theory
Does PhysicsOverlow consider pinging or private messaging specific users about a question inappropriate generally?