I don't follow you, how would a decentralized (and presumably non commercial) alternative be affected by this?
Besides the fact that legislation is typically a local affair Facebook would have an advantage versus another commercial player, and whoever wrote the law would most likely focus on those aspects that would have an impact on such players as well.
So for a non-commercial, decentralized and/or federated alternative this would be a huge boon, rather than that it would be killed in the cradle.
Are you anticipating a specific exemption for non-commercial operations? That hasn't been true historically.
For example, the notice and takedown rules in the DMCA require you to register an agent with the copyright office. There is no good reason for this when you have the info listed right on your site, but it makes a great trap for the unwary for anyone without the resources to hire a lawyer to tell them that, and causes grief for distributed networks where it isn't clear who should have to register and it almost certainly isn't the case that every participant actually has. Because the law came about as a negotiation between the big players with nobody even considering how it would affect small businesses and individuals or network architectures different from the then-dominant ones.
Or the regulatory nightmare in finance that has successfully destroyed a huge number of small nonprofit credit unions, or the medical industry where none shall enter without an entire law firm on retainer, etc.
People have just gotten used to it so they don't see what's missing. The idea that any individual or even pair of individuals could conduct a medical trial in today's environment is quite ridiculous, so nobody expects it to happen and then no one is surprised when it doesn't. But that was once the source of most of the progress in modern medicine before it was de facto prohibited by law.
It may be true that federated systems won't be affected because of a lack of enforcement, but that's cold comfort. Plenty will opt out of a system where they have to break the law in order to operate, even if the law is almost never enforced. And laws that everyone violates but are almost never enforced are inherently dangerous because they give the government a pretext to go after whoever they want for unrelated and pernicious reasons.
A little antitrust enforcement certainly wouldn't hurt -- stop letting Facebook buy its competitors. More competition would do two things. First, less people on each network, so less impact for each mistake. Second, more competition creates market pressure. It's much more problematic for Facebook to be able to do this and get away with it because they have a monopoly and users have no alternative, than for them to do this and lose all their users to a vibrant competitive marketplace, serving as a lesson to anyone who would do the same thing.
It also wouldn't hurt if the government would just overtly support decentralized systems. The internet itself, The Tor Project and most of the other decentralized technologies we have all came out of government research grants. Which we don't do nearly as much as we used to in this space.
We could also roll back some of the existing bad laws -- like SESTA -- that are already making it harder for small players to compete.
That's the point.
Imagine the result of all this is a piece of legislation that doesn't hamper Facebook in any significant way but imposes large fixed compliance costs on tech companies.
Facebook is huge, they pay the costs without even noticing. People who want to dethrone Facebook, or replace it with some decentralized alternative that actually protects privacy, are killed in the cradle by the new fixed overhead.
Facebook gets what they want and the Senators get to look like they've done something, meanwhile nobody pay attention to the fact that the same Senators are about to get a load of campaign money from Facebook.