I have some brilliant and enthusiastic friends in the science blogosphere who are putting substantial effort into building on-line venues where working scientists will create scholarly communities to engage in vibrant scientific discussion and commentary, as well as disseminate novel scientific information. As appealing as this may sound---and it does sound appealing in some respects---it is currently doomed to failure, at least in the case of the biomedical sciences.
These sorts of venues will be participated in only by very small numbers of actual working professional scientists, and dominated by hobbyists, bloggers, and other science enthusiasts. This is because of the way that scientific credit is allocated.
As things currently stand, scientists do not get any professional credit whatsoever for participating in this kind of activity, but they do get credit for publishing peer-reviewed research papers and lierature reviews and commentary, attending real scientific conferences where they present and discuss science with their peers who allocate credit, and traveling to other institutions to present their work in seminars.
This creates an extremely powerful incentive to not devote time and effort to on-line and real-world OpenConFooBlogSciWeb ELEVENTY POINT BAJILLION type activities.
There is a huge amount of inertia in the institutional structures of science that allocate credit in this way: grant review panels, hiring committees, promotion and tenure committees, award committees, etc. Regardless whether you think any of this is a Good Thing, there is no reason to think that it is going to change any time soon.