Whitelist based on HTTP USER AGENT

Some user doesn't want allow all country to access their website. But can be that for blocked users users need some service from blocked country.
In my case I don't want "United States" but I need facebook Opegraph.

Facebook use high number of ip address also in ipv6 that rsfirewall doesn't support so I can't use whitelist.
Perhaps I think that a large range of ip in whitelist can have performance problem with many concurrent users.

Allow all country IP to use some service is not a good solution but for now is the only way.

A solution can be to have option to skip ip block based on "HTTP USER AGENT".
I know that it can be spofed and open potential security problem but this option is yet used to skip other check like SQL Injection etc...

Any other solution is appreciated.

Thanks a lot.