First and foremost, I contribute to the group blog ETC-J - which by the way recently features an interview on open learning at P2PU: http://etcjournal.com/2011/08/11/open-learning-at-p2pu-an-interview-with-jessica-ledbetter/
Second, I also maintain a German blog about the Center for E-Learning at Ulm University.
Both blogs have a very different audience and purpose. THE CEL-blog serves as a way to document my work and allows my employers to see what I do all day... It also allows me to communicate with instructional design teams from other universities. I have their blogs in my blog roll and follow their RSS-feeds on netvibes.
The audience of ETC-J is clearly international. Readers / followers are people with an interest in educational technologies - or educational policies, practices .... etc.. Since the group blog is very diverse, I do not agree with all postings by other authors and I distinguish between the general audience and the adressees of my own contributions. Both are important to me, but matter in different ways.
At ETC-J, the editor in chief regularly monitors our page views. Currently, we have 5800 (July 2011). Our peak was 8100 this year in March. The general trend is positive: in June 2010, we had 2260 page views, so there is a substantial improvement in terms of outreach. In the end however, these are just figures: It is nice if the stats are high, but I do not need to have continuous growth to be content with the blog.
If I look at my audience from the perspective of my individual postings, I enjoy getting comments and being reposted on other sites, e.g. my ED-MEDIA conference reports can also be found at the AACE blog / website. That's a cool thing when you use a creative commons licence as ETC-J does. This helps me do understand who I am writing for. Generally, people who are interested in social, personal and open learning topics. I case of conference reports specifically for organizers and attendees of the event.
Why am I contributing to a group blog rather than having my own? Well, I do enjoy the proof reading and editing - a luxurious feature for non-native English bloggers.... Also, a group blog is great for not being pressured to post all the time. I have phases of high and low levels of contribution - depending on my workload. This does not mean that I only post on the blog if I am not busy. However, some tasks fit very well with writing blog posts - and others just don't...