Social networking website Twitter has confirmed that it has closed a "significant round of funding". Co-founder Evan Williams said in a blog post that the site had secured money from five investment firms. The site, which allows users to write and share 140-character messages, has more than 45 million users worldwide. Industry watchers have pointed out that the firm still has no way of making money. However, earlier this month the site revised its terms and conditions to allow advertising on its service. Some analysts believe Twitter could eventually be acquired by one of the established Internet companies such as Google Inc, Yahoo Inc or Time Warner Inc's AOL. Google CEO Eric Schmidt called Twitter a "poor man's email."