This tool is something that Facebook is trying to sell. To other companies. So you're missing the point with your superficial speculation as to the balance of power between Facebook and its employees.

For that reason, I'll keep the rebuttal short: laws are still valid, as far as I know. And this tool is blatantly illegal. Facebook employees also don't typically face starvation if they can't find a new job right away. And considering how rare unions are in the tech sector, and how common they are in, for example, automotive, my hunch is unionisation is more likely to happen at companies facing economic pressure.

Whilst it may be distasteful, it is entirely legal.

An employer cannot inhibit the formation or function of a union but are under no obligation to provide the facilities or platform to form one. What FB have done is akin to not allowing an organizer to use the company photocopier to print union leaflets

I think you’re wrong... It’s only legal if they prohibit all non-work discussions. So if they also block discussions about politics, sports, music, etc they can block talk about unions.

But they can’t target union talk specifically:

Companies may prohibit workers from talking about nonwork issues in work areas during work hours. ... In other words, a company can prohibit extraneous conversation, but may not prohibit only union discussions while allowing employees to talk about anything else they wish.

Source: https://www.nolo.com/legal-encyclopedia/shop-talk-rules-unio...

This is only true if they block all non-work discussions. Law explicitly allows union related discussion if other non-work related discussions are allowed.

While unemployment is high right now, I'm not sure it's affected most tech workers. I don't have numbers, so this is purely anecdotal, but almost all of my friends are in tech, no one has been laid off, and I just started job searching myself after being voluntarily out of the market since pre-plague, and I can barely keep up with interview requests.

I agree with the other reply to you, but to address your point directly, it is probably true that a lot of the frontline workforce can't afford to leave, but the people in leadership positions and very high-level/well-respected ICs still have options (including, given given FB's pay and recent stock performance, the option of not necessarily needing to work). That's a small group but each departure would be noticed and they'd have plenty of leverage.