Twitter Headquarters, San Francisco, United States

Twitter is ranked as one of the 10 most visited websites worldwide by Alexa's web traffic analysis. Daily user estimates vary because the company does not release the number of active accounts. A February 2009 Compete.com blog entry ranked Twitter as the third most used social network based on their count of 6 million unique monthly visitors and 55 million monthly visits. In March 2009, a Nielsen.com blog ranked Twitter as the fastest-growing website in the Member Communities category for February 2009. Twitter had a monthly growth of 1,382%, increasing from 475,000 unique visitors in February 2008 to 7 million in February 2009. It was followed by Zimbio with 240% increase, and Facebook with 228% increase. However, only 40% of Twitter's users are retained.
Adding and following content
There are numerous tools for adding content, monitoring content and conversations including Tweetdeck, Salesforce.com, HootSuite, and Twitterfeed. Less than half of tweets are posted using the web user interface with most users using third-party applications (based on analysis of 500 million tweets by Sysomos).