(FORTUNE Magazine) – Yesterday your grandmother started blogging. So why aren't more FORTUNE 500 companies joining the sphere? Some blognosticators predict that blogs are the future of corporate PR and that all 500-level companies will have them by 2010. But socialtext.net/bizblogs, which indexes FORTUNE 500 blogs open to the public, has found just 24, mostly operated inside tech companies. Fortune500blogs.com contacted hundreds of companies to find more and got just one response, a negative. Debbie Weil of blogwriteforCEOs.com chalks it up to uncertainty. "What's the ROI on blogging?" she asks. "Nobody knows yet." Blogging may still prove to be a great PR tool, but the 500 aren't on the blandwagon just yet.