If you resign from Facebook and don't tell anyone, have you really resigned?
Sure it is. If you boycott a company for doing something bad, you damage that company regardless of whether you are shouting from the rooftops or whispering it in a basement.
When outsiders see a given Facebook resignation, it does send a message, but even if they quit silently, Facebook's management sees employees leaving. While Facebook's exec team clearly doesn't give a damn about doing what's right, they absolutely care about keeping costs down and employee retention is a big cost for these companies.
Maybe this is better stated as "If you resign from a hugely visible platform, because it's bad, but you don't tell anyone, have you really maximized the impact of your resignation for good?"