When people try to consciously disrupt an entrenched business with massive inertia like Microsoft of 1990s, it fails. Examples include Netscape (hubris of "the web browser+internet Windows is the new operating system and MS Windows is obsolete") or Linux (cue recurring joke of "1995 is The Year of Linux Desktop!!!"). When your agenda is transparent, you just paint a huge bullseye on your back and fail.
Google/Alphabet for the first time in 2017 is about to pass Microsoft in revenue ($100 billion.) It took Google 19 years to do it. When Google started in 1998, they didn't know they would be working on a thing called "Android" in 2006/2007 that makes MS Windows less relevant -- especially for emerging web users outside of USA. Today, a mobile internet user with a cheap Android phone can check email (GMail instead of MS Exchange), look up encyclopedia facts (Wikipedia instead of MS Encarta CD), look up actors in movies (IMDB instead of MS Cinemania CDROMs), etc. Of course, that list includes Google creating a "launch pad" to other websites (PageRank search instead of MSN as web portal). MSN in 1998 was a small part of Microsoft's business. And things like GMail were not in Google's roadmap in 1998.
You can't disrupt Facebook head on with an obvious goal of disrupting them. Instead, you hit them sideways. You disrupt them by doing something else that doesn't look like you're disrupting them. For example, instead of creating a new "social network", you create a micropayments system that's easier than PayPal or Apple Pay. Or you create an entertainment platform with a "social" component as an afterthought. Maybe one of those non-social networks accidentally becomes a new focal point for social gathering and accidentally disrupts Facebook.
Put another way, the startup entrepreneurs that will disrupt Facebook are working in their proverbial "garage" right now -- but they don't know they will disrupt Facebook in the future. (That's because they're not working on an "obvious" social network.) It's good that they don't know that they're competing with Facebook because if they did, they'd trip over themselves with their ambitions and they'd start a battle they can't win. (E.g. Snapchat losing to Facebook's Instagram.)