The blog software that I'm using does allow comments (as an option for the blog creator) but, unfortunately, from my point of view they are a click away from the main posts. For an extremely popular blog, the approach with the Blogger.com software may have merit. The members of the blog initiate threads. The threads themselves are then a click away and organized by the subject line the member used in the initiating post. This creates a hierarchical relationship where members have more authority since they can post and comment while others can only comment. The hierarchy may be useful with large volume as a way of organizing the content.
But for smaller groups, the hierarchy is unfortunate. One can make everyone a member, if there is somebody to organize that. But that can be clunkly, because at least with the Blogger.com software membership is by invitation only. This is especially an issue in circumstances where individuals self-identify as members of the group and where they want to participate immediately after reading some posts they find engaging. There should be the option for the blog owner to allow new members directly on the blog homepage. Further, some people may not want to become blog members. (They may be ok in identifying themselves in their posts but not want to have that identity stored on the blog server.) It would be nice to address that concern by allowing anonymous comments to appear directly on the main blog page, interspersed with member posts as appropriate. Othere blog software may allows this, but in my brief surfing to find examples, I'd say that most blogs are not set up this way.
There is a related issue regarding the trajectory of group membership. Here is a little discussion thread on the realtive merits of blogs or forums for facilitating community. Though not a scientific study by any means the conclusion of this particular group, one that makes sense to me, is that smaller groups are better off with blogs while larger groups do better with forums. I believe this for precisely the reasons that I've discussed above. So one might envision starting with a blog and moving to a forum (or by other name newsgroup, web board, etc.) as the group grows with possibly an intermediate phase where both the blog and the forum coexist. And then if the group continues to grow it likely will fracture into subgroups, at which point it might begin again with some new blogs.
There is some open source software called drupal, a "content management system" which allows blogs, wikis, forums and a bunch of other functionality all in a single integrated environment. On their site they claim that they are making the plumbing for social networking. Perhaps they are. Certainly there is some appeal in having blogs and forums tied together.
Let me make one possible argument for why they are not. In essence what drupal and other open source developments that are similar are doing is separating out the software from the hosting. For small informal communities hosting is no big deal. Anybody can throw up a tiny web server. (I've got to be careful with the use of the wordy "anybody." I'm not sure I can. But I do think I could learn to do it.) But when the work of the community becomes more important then security and preservation of the content also becomes more important. At this point members of the community want to be sure the web host has made credible commitments to provide a robust service.
But what is the business model for that? If the web host is some volunteer organization on a shoestring budget, there may be problems with keeping the hosting service robust. Somehow, revenue must flow back to the hosting service to cover the costs of it. This might work where a commercial entity uses the community software as a way to engage its customers. In this way the hosting cost is a kind of a marketing cost and to the extent that is good for business it all fits together. But if the group is not in an obvious way identifiable as customers for a given for profit venture, then where do the revenues for the hosting come from?
Yahoo and Google and some other providers aggregate the software with the hosting and use their ability to command eyeballs as a a way to pool business that might want to advertise to the particular group. But they use their own software, not drupal. It is non-profits who are likely to provide the hosting for drupal. Yet the non-profits have to make a similar business calculation about covering the hosting cost. This suggest that relatively well to do non-profits will host while others less well to do likely won't unless the social networking is part of the core business of the institution.