To try to answer this question, I found quite a lot of data from 2003, some from 2004, most from the US. The sources and the methods used are disparate, but at the end some convergence emerges.
Active blogs
In June 2003, Blogcount estimated the number of active blogs (update during the previous 30 days) to be roughly 2.4 to 2.9 M. Most of these bloggers (1.6 M) use one of three Blog hosting services: LiveJournal, Blogger, DiaryLand. The same year, Jupiter makes an estimation of blog publishers to 2% of the total online community and of blog readers to 4%. Several Internet observatories put the total online US population at approximately 160 M. Blog publishers would thus be around 3.2 M, and blog readers 6.4 M. Both the publisher and reader communities are formed by people who have been online for more than 5 years. However, they differ in income (higher among readers than editors) and gender composition, readers are mainly male.
An estimation of the active blogs at 3 M +/- .5 finds support in the October 29th 2004 statistics from LiveJournal.com. On a total of almost 5 M, 1.350 M have been updated in the last 30 days; 870.000 in last 7 days; 340.000 in the past 24 hours. There are thus about 2.6 M active blogs. We also learn about some specificities of LiveJournal.com blogs. They are published using free accounts (98,1%), by a large majority of young female, between age 14 and 22, with a peak between age 16 and 19.
Very active blogs
It doesn't come as a surprise that the number of very active blogs, that is blogs that are updated daily, is much smaller. Drawing on the same statistics, we learn that they correspond to 340.000, around 14% of all active LiveJournal.com blogs. Recent statistics from Technorati (October 6th 2004) register an average of 300.000 posts a day in September and October, with picks of 400.000 posts a day for the 4 M blogs that Technorati tracks. It seems to me relatively safe to estimate the number of very active blogs in the 350.000 +/-50.000 range.
A much more conservative estimation of the number of active blogs however exists. It comes from Perseus' 2003 analysis of 3634 blogs (hosted by BlogCity, BlogSpot, Diaryland, LiveJournal, Pitas, TypePad, Weblogger, Xanga). Perseus puts the total number of blogs on these services to 4.12 M. The analysis reveals that 66% of them have not been updated in the preceding two months. This corresponds to 2.72 M blogs either permanently or temporarily abandoned. Of these, 1.09 M blogs had been created and never updated. The remaining 1.63 M blogs had been active for 4 months on average: 132.000 blogs for one year or more, with 13.600 blogs later reactivated. Looking at the 1.4 M blogs updated in the previous two month. The large majority are active blogs updated every 14 days. Among them, 105.000 are updated more frequently, at least once a week. Very active blogs updated daily are instead fewer than 50.000.
Who's right then?
Evidence from the Pew Internet & American Life Project national survey of 2515 American adults on the theme “Online content creation and publishing” supports the larger 350.000 +/-50.000 estimation of the number of very active blogs. The survey, which took place between March and May 2003, surveyed blogging as one of the forms of online content creation and publishing. Five questions dealt with three different levels of participation in blogging: creating and maintaining a blog; reading other people's blogs; reading and contributing to other people's blogs.
The report indicates that 2% of Internet users say they write a web diary or web log. This corresponds to approximately 3.2 M Internet users. The updating frequency varies from every few weeks (39%); weekly (31%); less often (20%); every day (10%). Very active bloggers are again estimated at about 320.000 people.
Blogs readers
Many more Internet users visit other people’s blogs. They are 11% of the Internet users, approx. 17 M people. This result is also reported by BizBlog, in a 2003 survey for an email services agency: 10% of the respondents (1691) described themselves as regular blogs readers. Going back to Pew, the blogs visited belong to friends (52%) and people never met (46%). Family blogs are visited less frequently (25%).
A more recent portrait of who blog readers are comes from blogads online survey taken by 17.159 blog readers. The majority of respondents were male (79%), over 30 (61%) and earned more than 45.000$ a year. Their digital life is quite extended: they read news online (54%), buy books (50%) and plane tickets (47%) online, make online contributions to causes or candidates (50%). One in five is also a blog publisher.
The blog readers who answered the questionnaire share a very critical view of traditional news sources: television (worthless 37%, somewhat useful 45%), newspapers (worthless 14%, somewhat useful 41%), magazines (worthless 12%, somewhat useful 42%), radio (worthless 13%, somewhat useful 39%). And, not surprisingly, a very positive view of blogs: 55% consider them extremely useful and 31% useful. More generally Internet is the main news source (54%).
They read blogs because they provide news they cannot find anywhere else (80%); because they appreciate the viewpoint (78%); because news come in faster (66%) and because they perceive blogs as more honest (61%).
Blogs readers and contributors
The Pew reports finds that, among blogs readers, 34% also post contributions to the blogs they read, what is equivalent to approx. 6 M people.
Conclusions
Converging results coming from a quite diverse range of studies of blogging suggest an answer to the "How many" question. We can estimate that there are between 2.5 M and 3.5 M active blog publishers, whose blogs are updated at least once a month. The very active publishers, those who write on their blog daily are between 300.000 and 400.000. There is much less data, and also less convergence, about the number of blog readers. Jupiter puts blog readers at 6.4 M, whereas the Pew Internet Project's estimation is almost three times higher, at 17 M. I found only one piece of data about readers/editors. The Pew Internet Project finds 6 M blog readers who post comments to the blog. Finally, there some indications about demographics. We learn that bloggers - publishers, readers, editors - have a relatively long online experience, of over five years. Some economic, age and gender differences have emerged between publishers on the one hand and readers on the other. As well as evidence that blog hosting services have developed distinctive audiences.
Where: US
When: 2003 - 2004