It’s made me realize that one of the bigger values of the iteration that Webmentions has over its predecessor pingbacks and trackbacks is that at least a snapshot of the content has captured on the receiving site. As you’ve noted that while the receiving site has the scant data from the pingback, there’s not much to look at in general and even less when the sending site has disappeared from the web. In the case of Webmentions, even if the sending site has disappeared from the web, the receiving site can still potentially display more of that missing content if it wishes. Within the WordPress ecosystem simple mentions only show the indication that the article was mentioned, but hiding within the actual database on the back end is a copy of the post itself. With a few quick changes to make the “mention” into a “reply” the content of the original post can be quickly uncovered/recovered. (I do wonder a bit if you cross-referenced the Internet Archive or other sources in your search to attempt to recover those lost links.)
I will admit that I recall the Webmention spec allowing a site to modify and/or update its replies/webmentions, but in practice I’m not sure how many sites actually implement this functionality, so from an archiveal standpoint it’s probably pretty solid/stable at the moment.
Separately, I also find myself looking at your small example and how you’ve expanded it out a level or two within your network to see how it spread. This reminds me of Ryan Barrrett’s work from earlier this year on the IndieWeb network in creating the Indie Map tool which he used to show the interconnections between over three thousand people (or their websites) using links like Webmentions. Depending on your broader study, it might make an interesting example to look at and/or perhaps some code to extend?
With particular regard to your paragraph under “Wait! Aren’t you researching Twitter?” I thought I’d point you to a hybrid approach of melding some of Twitter and older/traditional blogs together. I personally post everything to my own website first and syndicate it to Twitter and then backfeed all of the replies, comments, and reactions via Brid.gy using webmentions. While there aren’t a lot of users on the internet doing something like this at the moment, it may provide a very different microcosm for you to take a look at. I’ve even patched together a means to allow people to @mention me on Twitter that sends the data to my personal website as a means of communication.