I would argue that modern, prestigious tech companies use GitHub portfolios like admissions counselors use activity leadership and transcripts.
You're required to use all of your free time, and then a little more, to demonstrate that you have initiative and grit, that you don't need to sleep adequately or spend time on anything other than the "right" activities. Roughly analogous to taking hard classes and participating in lots of activities. Colleges don't want you "hanging out," and Facebook doesn't want you sailing or running a community theater group or raising kids or something, they want you sharpening your skills. Both employers and colleges are basically looking at, "how many hours per week do you spend on impressing us?" Proving that you ran a chronic sleep deficit and had no more than a couple of hours downtime outside of what they want to see gets you past the first cut.
Then, to make the next cut, the quality of the code you write needs to be exceptionally high (indicating that you really invested the time and that whatever your cognitive function declined to was still great.) Is your code elegant and clever? Did you write painstakingly thorough tests? Is everything perfectly documented? This is roughly analogous to having stellar grades in your AP/honors/advanced classes.
Next, as a tie-breaker, they look for interestingness. Were the apps you wrote/clubs you started innovative? Did they merely demonstrate that you are generally good at things, or did they actually make an impact on the world? (There is generally a field on the applications to elite colleges for you to submit your published, peer-reviewed scientific papers. As a 17-year-old.) That may fast-track you to the top, but lacking it won't necessarily kill you unless you're elbowed out by people who do have it.
And finally, the interview/essays. Do you write exceptionally well? Are you well-spoken? Do we like you? Do you seem like one of us? Colleges use this to keep the common threads they're interested in (at my alma mater, it's no accident that every single one of us identifies, whether obnoxiously or quietly, as an intellectual, and as much as we complain, being surrounded by people like that is kind of exactly what we're paying for). Then they create the distributions they're looking for in other elements of personality (i.e. we have to reject some bassoon virtuosos to make room for the virtuoso cellists, and we're going to need to sprinkle in a few outgoing socialites to keep the awkward nerds from killing themselves.) Though I guess some employers and possibly colleges are only going for homogeneity - the IBM of old is notorious.
It seems that basically every college and/or employer is oriented towards behaving this way, but the defining characteristic of the elite communities (whether they are corporations or universities) is that they are closer to filling their slots without "settling" on anyone.
The composition of applicant pools between institutions is not random or uniform. Certain schools only have a profile in more-intellectual circles, and I imagine it's the same with tech companies. A school may have a 2% acceptance rate among an applicant pool whose median ACT is 15. Or a school may have a 75% acceptance rate because only the overachiever children of academically elite families and school systems have heard of it. So eliteness is not necessarily acceptance rate, but more like "lack of settling."
Recruiting activity may be focused on places where smart people are likely to be more dense, but it does seem that some companies are starting to play "admissions counselor" for themselves when evaluating applicants, looking at GitHub rather than your degree.