The title feels clickbait-y. It should be pretty obvious why there were only 500K downloads- it's only available on around three devices. I'm interested to at least take a look at it, but I can't. As for the reviews, many of them are of the "I hate Facebook" variety, but others seem to state (correctly) that it need some work. Given Facebook have committed to monthly releases, I don't think this is the end of Faceboo…
As comparison, Facebook for Android app has the average rating of 3,5. So I guess the 2 stars should be taken as an alert for the Facebook Home developers to keep working on it.