First of all, RSA is nowadays usually only used for authentication when used in TLS. And authentication is performed using signature generation, not encryption. Sometimes this is called e.g. SHA-256 with RSA encryption but that's a kind of misnomer. Starting with TLS 1.3, RSA encryption will not be used at all anymore.
But mainly the client certificates used by Google use Elliptic Curve cryptography (ECDSA) in general. So the key agreement - which determines the symmetric keys used to encrypt the traffic - is performed using ECDH key agreement and the authentication is then performed using the ECDSA private key on the server and the certificate containing the public ECDSA key send to the client.
Previously RSA encryption of the master secret was commonplace, but that doesn't give you forward secrecy, so it is being phased out.
Here is a picture of the security provided by the Google site. RSA is only used to verify the server certificate, not to secure the connection itself. Similar story for Facebook.
Note that browsers that do not support ECC cipher suites may still use RSA certificates.
The reason why the connection is secured is given by the other answers.