You can import a key on your device by sending yourself a PKCS12 encrypted archive as an attachment in an e-mail that you can read on your iPhone or iPad. The PKCS12 attachment must be password protected, and the filename’s extension must be changed to “.synapsid” or “.synaps1d” otherwise iOS will interfere and add the key to the Mail app. If you export your key from Synapsid it will automatically attach a compatible version that you can then re-import in Synapsid on a different device (this allows you to use the same key on all your devices).
Using an S/MIME key from a certificate authority has some advantages over the self-signed key Synapsid automatically creates. The major difference is people won’t receive a warning about an unverified self-signed certificate when sending them e-mail.
Synapsid supports S/MIME keys from any issuer. You’ll need to find out your Facebook e-mail address which is typically [username] @ facebook.com or [user identifier] @ facebook.com if you never setup a shortcut url for your profile. You could also send a message from your Facebook account to your e-mail address.
Most certificate authorities issue S/MIME credentials, which one you use is up to you. I use SmartCom which provides free S/MIME keys and certificates.