A lot of social media sites will try to get you to sign up to save/bookmark posts on-site or in-app (even Lemmy!), but if you’re viewing them via browser you can just save a bookmark to your browser.
The perks to this are: you don’t necessarily need to make more accounts, your browser bookmarks can be renamed & tagged & organized into different folders, and you can search your bookmarks!
A lot of on-site/in-app bookmarking doesn’t provide those benefits, at least not without trying to make you pay up.
To expand on this: on mobile you can also use the share feature many sites provide to share either to yourself on certain instant messaging apps (afaik WhatsApp and Signal allow this) or, even better, to some note taking apps.
Just watch out for whether or not the target note taking app is cloud based or saves files locally, else you’re running the risk of moving the problem OP has mentioned from the social media app to the note taking app.