Twitter Inc., the fast-growing microblogging service, was inaccessible Thursday morning, struck by a "denial-of-service" attack, the company said on its status blog. "We are defending against a denial-of-service attack, and will update status again shortly," the company said in a post shortly before 11 a.m. EDT (1500 GMT) Thursday. In an update to the blog post, Twitter said the site is back online, but that the company is "continuing to defend against and recover from this attack." The site will remain slow for users attempting to access it while Twitter attempts to recover. "We're working to get back to 100% as quickly as we can," the company said Not surprisingly, posts related to the cyber attack have soared to the top of Twitter's most popular topics queue now that the site is back online. Social-networking giant Facebook Inc. also had network issues this morning, with some users reporting that certain features have been slow or not