(Reporting and Writing by Laura Isensee)

    Still not sure about Twitter?
   More than half of Fortune 100 companies are using the wildly popular micro-blogging site, according to a new study, adding to growing buzz in the business world that Twitter hopes to convert into revenue this year.
   

Twitter trumped other social media as the online tool of choice for the Fortune 100 firms, according to the study by public relations firm Burson-Marsteller and its digital media unit Proof.
   

Some 54 percent of them regularly send out “tweets” or messages of 140 characters or less. The companies primarily use Twitter to blast news; for customer service; to announce promotions; or for employee recruitment. 
  

 Blogs hold the No. 2 spot, with 32 percent of the companies regularly posting to a branded blog.
    Social networking site Facebook came in third. Twenty-nine percent of Fortune 100 companies have active, consumer-focused fan pages on Facebook.