Here's a list of all the top sites used frequently used everyday hosted on Linux. The list is based on Alexa Traffic Ranking. Here it goes.
1. Google (#1 Alexa Ranking)
Everyone knows what is Google. We all use Google as a search engine. Google now also offers tons of services including Apps. Today we also use Google as a verb! "Google It!"2. Facebook (#2 Alexa Ranking)
Facebook is by far the largest and most popular Social network on the Planet. It is used Worldwide and lets everyone share practically anything with friends on their network. Facebook is also equipped with Apps, while letting developers program their own.
3. Youtube (#3 Alexa Ranking)
A Website owned by Google to share videos, now even in HD and 3D.
4. Yahoo! (#4 Alexa Ranking)
One of the most popular Internet Portals and Service Providers across the World.
5. Wikipedia (#6 Alexa Ranking)
The Internet's Largest Worldwide Encyclopedia!
6. QQ.com (#7 Alexa Ranking)
China's most popular Internet Portal
7. LinkedIn (#8 Alexa Ranking)
The Social Network for Professionals! Includes Employers and Employees, and is based on work experience and Job performance.
8. Twitter (#10 Alexa Ranking)
An online social networking and microblogging service that enables users to send and read "tweets".
9. Amazon (#11 Alexa Ranking)
The Largest Online Marketplace.
10. Blogspot (#12 Alexa Ranking)
Pretty much sums up all Blogs hosted on Blogger!
11. Wordpress.com (#15 Alexa Ranking)
Same thing but for Wordpress.
12. eBay (#22 Alexa Ranking)
A Marketplace and a place to buy and sell stuff online. Maybe even Auction them
13. Tumblr (#26 Alexa Ranking)
A feature rich and free blog hosting platform offering professional and fully customizable templates, bookmarklets, photos, mobile apps, and social network integration.
14. Pinterest (#27 Alexa Ranking)
Pinterest is an online pinboard: a place where you can post collections of things you love, and “follow” collections created by people with great taste.
15. Paypal(#36 Alexa Ranking)
Online payment service for individuals and merchants. Allows users to send money and bills to anyone with e-mail. Hides the user's actual Bank details and Card numbers.
As you can see, Linux servers are hosting the world's largest Search Engines and Web Portals. Also, it seems that when you visit a blog, you have a 99% (or maybe a bit less) of it being hosted on Linux. The Most popular Blog hosting sites are Blogger and Wordpress, and they both run on Linux. For people running their own servers, Wordpress is also available on Linux Servers.