First, unionizing is not simple. One needs a majority of workers in agreement of forming a union and voting on it.
Meanwhile, anyone who stands to lose money from better pay and benefits from the formation of a union -- anyone in management and the executives -- will likely try to stop any efforts, mainly through propaganda: "Unions are not good for you!"
With a company as big as Facebook and management likely monitoring everything its employees do, unionizing will likely be difficult, especially if people are not educated on the benefits of a union.
Contrary to popular belief, you cannot get fired for joining a union.
> With collective bargaining power, they could get much better work/life balance.
That is assuming they are unionized, which would take a while.
Facebook working conditions are not horrible, and for people who don't like it there is a great option of not working there.