"definite must"? If I find a successful developer, for some value of successful, that doesn't blog, would that prove your friend wrong?

You need to decide what your goals are. Are you striving to optimize your career for followers/notoriety? Than yeah, having a blogging/twitter/social media presence is a must. If you're striving to optimize your life for maximum profits and earnings, than having an online presence can help, but is probably not a must. If you're striving to become a great developer/programmer and are doing it for the love of the craft, then spending time blogging/twittering/etc will probably get in your way.