Facebook has about 800 million Facebook users. If you look country wise, US is on top, Indonesia, second and India being the 3rd with 30 million users. The internet users in India is growing rapidly and very soon India would be on the top of the World in terms of the number of Internet users and even Facebook users.
Facebook is used by most of the internet users, could be for personal use or even for marketing and selling their products. The reason for such a big success for Facebook and Twitter was because the way these services can be accessed. You can read / update status using a mobile phone / tablet / PC. The majority of Facebook users use mobile devices to use services like Facebook / Twitter.
