Twitter experienced site-wide downtime for the third instance in as many weeks on Thursday morning, as many users were unable to load the site via the Web or mobile devices. “Some users may be experiencing issues accessing Twitter. Our engineers are currently working to resolve the issue,” the company said via a site status blog. It’s disconcerting as the site grows into popular mainstream use; Twitter needs to distance itself from the early “fail whale” days if it wants to prove reliable to both consumers and advertisers alike.