In this context I would guess that "significant" refers to the size of audiences (i.e. it means more or less the same as "large") and "strategic" refers to their importance for whatever purposes are promoted. For example, it might mean that local journalists likely to report on news related to the US are particularly important (and so "strategic") audiences; they should have access to accurate information so that their reports in the local press (which will be "consumed" by a wider audience) are complete and reliable.