JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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Unfortunately, Facebook does not support Tabs on mobile, which is why you won't see the form when viewed on mobile.
A workaround you can do is to add a direct form link as a Contact Us button on your Facebook page.
Another alternative would be to embed the form with the 'Static HTML: iframe tabs' Tab App: Adding a form to Facebook (Custom iFrame App)
This app provides you with a special tab link for mobile devices.
Then, you can use that mobile Tab link as the Contact Us button link.
If you need any further assistance, please let us know.