Agreed. Note however, that it's not only open software that is a niche. There are many closed services as well that don't get any traction. For example the several email providers that don't read your private mails. They're a niche, too and people keep using GMail. Or other shops than Amazon. People often just use the dominating service. That doesn't really have to do anything with open software or anything. I think it's a bit of convenience and mainly people use what they're familiar with.

Unfortunately I don't know much about Friendica. I heard it has privacy, friend circles/groups, different post types, a feed and interconnects with other platforms. But I've never used it myself, because I don't really do social media except this platform here (Lemmy). I think Mastodon is very popular, but it's not alike Facebook at all. Other than that I can recommend writing a blog or having a website... But you can't really share family photos there. Or one of the Linktree clones, so people can at least find you and get your contact details if they want.

And yes, I also don't think these platforms are immutable. It's just impossibly hard to overcome. But all the current services have started somewhere. And this isn't the early internet anymore. It's a different story for Google Search, they've been here a long time. But all the Facebooks etc had to outcompete someone and overcome the network effect. And they did so successfully.