I can reproduce this exactly as you describe it. It's only on
the news feed page (Home), which has a lot of auto-refreshing
(Ticker), so I suspect something in this field. If you switch to
another tab, processor load goes down immediately, and if you
scroll on FB, load is high in any case, so you don't really notice
the problem other than looking at Activity Monitor, but it's there.
I've no idea what the browser is doing. Last version that doesn't
show this behavior is 19.0.2.