https://lifehacker.com/text-your-members-of-congress-with-resistbot-1794604660
Resistbot is a free service that turns your texts into a formal letter, and then faxes it to your elected officials. You get a copy of the letter, and a notification when the fax goes through. Although it takes its name from the idea of resisting the current administration, its a straightforward tool that you can use to voice your opinions, no matter what they are.
Phone calls are still the most direct way to get a staffers ear, but this text-to-fax service has a few advantages on your end: you can speak up any time, the bot will keep trying if the lines are busy, and you dont have to work up the nerve to talk to a stranger. Some members of Congress still have actual fax machines, while others will receive your message as a sort of glorified email. Either way, if your contact information is on the letter, it will be counted along with other messages from constituents.
You have to share some personal information with Resistbot to use it most effectively. When you first text resist to 50409, the bot asks for your full name and zip code, and it sends your first message to both of your senators. After that, if you give the bot your address, it will look up your congressional district so you can send messages to your House representative as well.
Over time, you can unlock more features. I can now direct a letter to just one of my three members of Congress. And I gave the bot an image of my signature to make my letters look even more personal. The bots creators promise to only use your information for the stated purpose (such as signing your letters, or looking up your district) and for other compatible purposes. They also promise that they will never sell your data. You can find their full terms and privacy policy on their About page.
You have to have a Facebook account with its Messenger service activated. Then, you either text RESIST to 50409 on your mobile phone, or go here and do it:
https://www.messenger.com/t/resistbot You sign in with either your phone number or your FB screen name and your password.
It gives you step by step instructions after you text RESIST. For some reason on my phone it did not, but it might be because I never use my phone texting for any FB stuff. IIRC, you can send a fax to the President, your Governor, your Senators and your Representative.
I'm not sure if you can do it right on Facebook. My guess is, you probably can.
eta: also here
https://www.facebook.com/resistbot/ and at
https://resist.bot