Why are you convinced of that? Unionized protests frequently accomplish institutional change - why do you think Facebook or Twitter would be exempt? If anything a unionized tech force striking would have more bargaining power than other groups since they are educated, specialized, and difficult/expensive for either company to replace en masse.

I agree that unions could be effective, but I also doubt that Facebook/Twitter employees could ever unionize. And for anything less than a full strike, leadership will just ride it out.