As a blogger for nearly five years I have seen lots of people come and go from blog writing and many who give up on the medium do so because they don't have enough of a work ethic to make their postings regular enough for the development of any kind of audience. Without regular readers and commenters its hard to keep writing a blog. So many bloggers give up before they get over this stumbling block.
The other reason that so many blogs are started but fail to continue is the simple fact that blog writing, like all writing, is a craft and it must be learned but there are no courses on blog writing so most practitioners are stumbling along in the dark seeking a niche and an appropriate 'voice" Many fail to find either and they give up.
The final characteristic of a "successful" blogger is a "work ethic" that enables them to post on a regular basis, daily is best in terms of keeping/ building a readership but a couple of times a week can "work" but anything much less than once a week is really a waste of time for readers.
I really don't think that Facebook or twitter can compete with blogging in the way that it can canvas the issues de jour or provide a forum fro discussion or debate. I use Facebook but I have no expectation that it has much more than a phatic function.
Twitter is obviously very attractive to those individuals who are welded to their mobile devices but my survey of various twitter streams shows that most of the communication is utter garbage unless you want to know when someone is on the dunny or off to have a wank and it has no value if your interests are not scatological

Did CS Lewis beat Vannevar Bush? In "That Hideous Strength" (1943), he describes an electrical notice board or "Pragmatometer" in a new scientific institute:

"There are to be forty interlocking committees sitting every day and they've got a wonderful gadget - I was shown the model last time I was in town -by which the findings of each committee print themselves off in their own little compartment on the Analytical Notice Board every half hour. Then, that report slides itself into the right position where it's connected up by little arrows with all the relevant part of the other reports. A glance at the board shows you the policy of the whole Institute actually taking shape under your own eyes." (p.382, 1989 Pan ed.)