The Class B stock has 10 share voting rights per share of Class B. This means that out of 10 billion class A shares the class B shares would still control the company with up to something like 9 billion shares of Class A outstanding shares. Currently, we are slightly above 3 billion shares outstanding. This means they could further dilute the OS by selling up to 6 billion more class A shares without losing control.
My guess is they won't go too much further because it will reduce their flexibility to do business. That being said I am sure they will continue selling/distributing class A shares for some time until incoming revenue streams pick up...if they ever do.
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