Look, my grandmother is not going to browse hackernews so she can download the facebook.
If she could search facebook on google and get a big old < Install > button next to the result, then maybe. But probably that won't happen on Google since they are incentivized not to for their 30% cut
In fact, google does return facebook as the first result when you search for "facebook"[1], and as for the install prompt that's something that facebook itself can show.
