If you have 8-10 dedicated authors, each of which commits to posting once a week, you can have a highly successful non-profit blog. The quality of authors is impressive on Times and Seasons. It’s a mix of Harvard lawyers, philosophy professors, and other really smart people.
I would argue that these don't have to be bloggers from the same organization, although that's one route to go. I could see having a group blog made up of bloggers from different organizations that share the same mission--for example bloggers from environmentally-focused organizations. I could also see a more "horizontal" blog--bloggers who address poverty from different angles with representatives from nonprofits that deal with homelessness, low literacy, job training programs, food banks, etc.
Education seems to do a lot with group blogging with some good examples here, here and here. Can't say that I've seen a lot in other arenas, so I'd be curious to hear of others that might be maintaining group blogs.
Having a group blog like this lowers the pressure on any one person to post daily. Each person commits to blogging once every week or so, depending on the number of people. They would also commit to interacting with each other's posts, by responding to what others have said and through comments, which would increase your blog's interactivity.
A good way to try out blogging without having to make the full commitment of running a blog on your own if you can find other organizations willing to join you. It's also another strategy for creating value in a nonprofit network as the organizations forge a more common identity through the blog and are able to create a synergy of ideas and connections.