can't sign in to facebook on my ipad

Contact Facebook for assistance with their iPad app.

There are known problems and Facebook has to fix

them - not an Apple issue.

Have you tried? Facebook and other social media platforms should be accessible, from any computer, using a web browser.

Some browser software may be too old to access some online resources and services - and may require update. If you are using an older model iPad (the tags appended to your post suggest that you have an iPad running iOS9 - iPadOS 16/17 being current), updating software to a sufficiently recent version may not be possible. In such circumstances, replacement of the iPad with a much newer or current model will be your only option.