A personal anecdote: people were willing to fly thousands of miles to come to my wedding, but (almost) nobody wanted to use the site/app we chose to avoid using Facebook to plan the event.
I think the only option is to make it seamless for Facebook users--let them log in with FB, use FB Messenger to send them updates, etc.
If you can't communicate your service to others, how do you expect others to use it? The best you can do is make it generic and easy to share however people want.
Supporting SMS and/or email means you can share this service in the most basic means available, and as long as Social Network X supports link sharing, Stuff will support Social Network X.
You just need to cc the address in an email and it sends invites to all of the people in the email list. I used it for an event of about ~30 and most people added their RSVP. It has chat too, but that wasn't as successful. Email is obviously not the 'coolest', but this approach at least reduces friction. I spent about a month thinking about / building this and would be happy to chat.
Edit: kinda funny we both chose pink :)