I guess it's worth clarifying what exactly you're talking about. If you want to share images and text with a small group of people, okay, that might be useful in some cases. But that's not the use case that Facebook users have in mind - you're setting the bar almost comically low and the impact on the actual landscape of the web will be exactly zero.
If you mean something that can make a splash in the social media space to address the "user agents on Facebook" problem, color me skeptical about the prospect of competing on useful features with the Facebook behemoth while fighting the complexity up and down the stack of making everything decentralized and trying to make it friction-free for casual users to run, with no funding from ads, and starting out at square one on network effects. Yes, Mastodon is a possible counter to this line of argument, but Twitter is so stagnant and their product is so simple that I feel like it's almost a unique case. And for most people the Facebook use case includes being able to find all their real-life contacts; by that measure even Mastodon would fail.
You can be as skeptical as you want to be. Even assuming this idea was intended to "make a splash" (it isn't), these sort of comments make no difference whatsoever. We have all seen how HN commenters have criticised ideas that, rightly or wrongly, later went on to become successful businesses. The thing is, this is definitely not intended to be a business. It is just some software that exists and that works. If it works for me, then it is "successful". There is no budding "founder" to shoot down. Just a user with some software that works. The assumptions of "make a splash in the social media space" and "competing with Facebook" are all wrong.
The business of Facebook is not the "comically low" bar of sharing text and images with friends and family. An overlay solution that avoids sending traffic to a third party server is not "competing with Facebook". However it could be used to avoid Facebook which is the point we are discussing here. The fact that only a small number of people actually use a solution does not mean it is a "failure". If the software is relatively small, compiles fast, runs on different OS and architectures, stays available for download and reliably works as intended, then to me it is "successful". The way I evaluate "success" and "failure" of a software is probably different from many commenters/readers.
And, secondarily to your main point, the idea that "sharing images and text with a small group of people" is some weird niche case that Facebook users don't care about is... pretty off-base. I'd say it's the main use case for the majority of Facebook users.
If you mean something that can make a splash in the social media space to address the "user agents on Facebook" problem, color me skeptical about the prospect of competing on useful features with the Facebook behemoth while fighting the complexity up and down the stack of making everything decentralized and trying to make it friction-free for casual users to run, with no funding from ads, and starting out at square one on network effects. Yes, Mastodon is a possible counter to this line of argument, but Twitter is so stagnant and their product is so simple that I feel like it's almost a unique case. And for most people the Facebook use case includes being able to find all their real-life contacts; by that measure even Mastodon would fail.