I think both of those objections only partially convey their real meaning. "I don't have time to blog" = "The time it would take me to blog is worth more to me

I think both of those objections only partially convey their real meaning. "I don't have time to blog" = "The time it would take me to blog is worth more to me than the ROI I'd get from writing a blog." and "I don't know what I'd write about" = "I don't know who'd be reading my blog so as to make it easy to know what to write about." Facebook and Twitter address both objections by giving users a well-defined, sometimes really big audience. The idea of writing stuff they think no one's going to read isn't very appetizing to most people.