I don't understand need for a two forms of communication rule - if FB can be accessed from your phone, why should the leader send another notice to your phone via text? My thought here is an internet-based form of communication can be accessed by any internet-enabled device, so it can be accessed easily for most people. If your phone falls in a puddle, you're not going to get a text, but you can still get a FB message. If your computer dies, you can still get a FB message off your kindle or phone.
PS: this is mainly coming from my point of view that I do not need nor want 2-3 notifications for every thing. I have 3 kids who have multiple activities each, plus my own meetings and appointments, I don't want to drown in notifications of the same change/activity. I hate that my dentist sends me an email, a text, and forces a google calendar appointment onto my calendar (that ends up duplicating the appointment I already put there).
Because stuff happens. Web pages crash, internet goes down, power goes out, etc etc. There are eleventy billion ways to communicate. And every single one has a potential for failure.