Facebook Group for changes due to Covid19?

I don't think that social media is ever the place to make wedding announcements. I would send out invitations as usual, unless you know that you'll be postponing before it's time to send them, and then reach out to everyone over the phone if you need to push your date back. Do you have an idea of when you plan to make your final decision?

Yeah, I would find that super odd then. So you would just purposely send out invitations with the incorrect date? If printing new invitations is out of your budget, I would DIY some business card sized inserts with the new date and include them with the original invite.

I think given the super weird circumstances, noone will care too much how you handle changing your date. I agree with Caytlyn that one route might be to send your original invites with a small cheap insert (you can get 100 business card size inserts on Zazzle for like $20). But if Facebook is a really common way to communicate for your family/friends, I think using a private group would be fine too. Another option could be to use entirely digital invitations, which are available through paperless post and other websites, to create digital change the dates for much cheaper than mailing them out.