I agree that Zigtag does many things fairly well, and even better than that. I really feel sorry that it (and the Common Tag approach in general) hasn't succeeded yet in attracting a reasonably large community of users. How do you feel about it? Do you think it has failed or is it still breathing a little bit? Do you have a general theory of what it did wrong?

About your last paragraph: yes, there are some times when I wonder whether groups are really needed. Maybe a simpler architecture based on tags alone would be sufficient for most purposes.

Some verbs I like in this context: share, track, filter, structure, organize, map, list, recommend, create, innovate, learn, debate, decide, choose, motivate, contribute, help, collaborate, open.

And if I had to choose three at this stage, I think it would be: "filter, structure, debate", but this doesn't do justice to co-creation...

About "talkative bunch": this item got 2 views so far, I think it explains it. But I agree it could lead to a rich discussion.

Yes, this is particularly annoying in Twine, where loading the metapage takes forever. I wouldn't mind if Diigo took me to the annotated page, with the highlighted content and a link to the original page. I don't find the current implementation (with a horizontal split of the linked page) very convenient.