The number of tweets posted on Twitter hit the 10 billion mark this week, highlighting the very rapid growth of the micro-blogging service which was launched less than four years ago. The only disappointment perhaps for Twitter enthusiasts is that the San Francisco-based company was not able to reveal precisely which of its users sent the 10 billionth tweet, due to privacy regulations and the fact that the tweet was posted on a so-called ?protected? profile. This simply means that the individual user?s tweets were only visible to a small handful of followers, rather than the broader public. The number of tweets generated is likely to increase even further with Gigatweet?a website dedicated to tracking Twitter-related statistics?estimating that the micro-blogging service may pass the 20 billion mark as early as July.<br /> If the number of headlines that news of the 10 billionth tweet generated serves as any indication?including in major publications like The Telegraph?this prediction might be accurate. But it need not serve as a surprise that major British papers are paying such close attention to Twitter?s fortunes, considering that London residents send more tweets than users in any other city in the world. Twitter use is also spreading among major companies. For example, a total of 57 FTSE 100 firms already use Twitter in order to stay in touch with their clients.<br /> According to Twitter?s own statistics, users now send nearly 50 million tweets each day, with an average of 600 posts per second worldwide. While all of this is encouraging news for Twitter, the micro-blogging site still has some work to do in order to overcome its two larger rivals, namely Facebook and YouTube.