In much the same way that cash as the primary currency has been displaced by cheques, debit and credit cards, not to mention ecash options, so are communication vehicles increasingly fractured.
The choices for online communication vary widely from: old style enewsletters, to blogs, to 140 characters on Twitter, to more social media like LinkedIn and Facebook communities, YouTube and wikis. And we should expect that more options will develop and emerge.
Note that I deliberately did not use the term “marketing”. Although many of these media are used for marketing most of these media are much more interactive in nature. Most allow for comments at least and many for ongoing dialogue. This results in developing ‘relationships’ and this presents entirely different opportunities and challenges.
As far as cost / benefit analysis: the jury is still out.