Our theories of time travel are just that, theories. (for the population) It could be that things are very resistant to change. We don't really know. It does seem that introducing something new would have an impact later on, but instead of a series of dominoes, perhaps it's more like diminishing returns at each ripple caused. People could potentially be posting stuff from the future all the time, but we lack the respect of those things in order to allow them to influence us.