Don't know why you're getting downvoted. It's a fact - when you keep scrolling Facebook, it eventually slows to a crawl. Want a proof? Just go to a profile page of any of your frequently-posting friend and keep scrolling down.

Also, given how fast you can scroll back to the beginning, I don't buy the DOM-replacing technique. I've never seen it working on the desktop website.

Both Tumblr and Facebook do indeed replace off-page content in the DOM with placeholders. A chrome extension I am making that makes an offline copy of a page will make a snapshot of the page content with the hidden DOM elements in place, as can be seen here: http://i.imgur.com/bwGA0tp.jpg. Also - previously I worked at Tumblr, so familiar with the code base driving the dashboard.

While this technique does save on immediate memory consumption, I am sure that maintaining the stack still causes eventual slow downs.

Could it be because older content is not cache prioritized as much as new content? I would imagine anything after 1 day gets accessed much less than new content.