You make great additional points on this topic. @Don brought up the similar issue with people are just conditioned to ignore advert space on websites now and you make an even more specific point that a lot of companies/organizations now block adverts for security reasons as well. Thus making it a waste of money to attempt to target an "audience" that is already rate limited.
Your second point on single provider is a good one from a contracting practice and practical practice as well. Single points of failure in emergency management is obviously a VERY bad thing!
Finally, the "digitial divide" point is a huge discussion right now in #SMEM. A lot of organizations seems to forget that for quite a few emergencies the one thing you are practically 95% sure you are going to lose is power. This is one of MANY reasons I constantly support social media as an OPTION for emergency management, but not as a primary solution in any way, shape, or form. Focus needs to remain on solid emeregency communications processes and procedures to ensure that proper validation of information is taking place along with citizen engagement, but remembering that social media is simply another communication platform in the tool bag.