At this time, triggers have to be something that "happens", rather than something that "doesn't happen". In order to do what you are suggesting, the application would (likely) have to constantly loop through all users trying to determine if they haven't posted over a period of time, and then initiate the campaign if so. This would be quite taxing on a moderately-sized site, let alone a large site. There could be clever ways around this, but they'd have their own drawbacks.