There is some really, really delicious irony in that complaint... perhaps if Facebook paid for higher priority mail delivery, their emails would deliver. :)
I've always found these "email your congressmen!" campaigns to be largely ineffective and even counterproductive. The FCC opened net neutrality for public comment and received a similar response, i.e. hundreds of thousands of form emails sent to them. The emails bury any signal under gigabytes of noise. The only result is that regulators will ignore all emails, not just the form letters. The form letters are basically a regulatory DDOS campaign.
Add to that the fact that the campaigns are orchestrated by large players in tech, e.g. Facebook, and they lose all credibility. A megaphone and an agenda should not be sufficient tools to subvert public discourse.