Every site is different. Varnish is great for a particular fairly common use case, but it's not an all purpose "make site go faster" button. For example, putting varnish in front a CMS is usually a fantastic idea, or anything that has a high ratio of reads vs. writes and serves the same pages to multiple users. That could be anything from a content blog or site, an online store, etc. However, for other types of sites it doesn't make as much sense. A site like facebook or twitter would gain almost no advantage from it, since the overwhelmingly most common use case is for every single user to receive different pages on every single visit. Similarly, it doesn't make sense for search engines, or for web mail apps, etc.
Also, most really large sites have probably already developed some other method of caching if it suits their site needs, so it wouldn't make sense for them to switch over to varnish all of a sudden.
Facebook uses Varnish, so does Twitter. They use it where it makes sense, where reads are high and content is less dynamic. To say they'd gain almost no advantage of it is oversimplification as they have various requirements and some of those do indeed benefit from caching.
Also, most really large sites have probably already developed some other method of caching if it suits their site needs, so it wouldn't make sense for them to switch over to varnish all of a sudden.