The number of blogs and the use of blog readers rose rapidly last year — but a majority of Americans still don’t know what a blog is. A report by the Pew Internet and American Life Project, called the “State of Blogging,” discovered that readership of Web blogs — essentially, Web-based diaries — spiked 58 percent last year, with 27 percent of Internet users, or 32 million people, saying that they read blogs.