Advantages
- no $ cost
- no dev time investment
- no hosting cost
- maybe existing adoption to merge with
(hrm looks like no subtopics in Discourse?)
Twitter hashtags - effectively an in-group SharedVocabulary
Sometimes you can leverage a “hot” existing/fresh hashtag by using that hashtag plus your in-group hashtag.
Twitter lists
I made one for this group
https://twitter.com/i/lists/1275414236383109120/members
But I haven’t maintained it. If there’s an “official” member directory, ideally including people’s Twitter handles, I’ll do so periodically. Is there one?
Lists are good for in-group reinforcement, but don’t necessarily spread well.
Google CoOp CustomSearchEngine http://www.google.com/coop
You can make a search form that’s limited to a whitelist of sites.
I think you can do subsites, as long as the URL has a certain consistent pattern. (Can you do this for individual twitter users? Dunno.)
I made one yesterday for a tiny group of sites, probably not of interest to this group. https://twitter.com/BillSeitz/status/1289743892804329472
Good for reinforcing trustworthy sources.
I don’t know if there’s a limit on the number of sources you can add.
That’ll likely lead to a useful list. On the other hand, these properties can lead to many limitations or loss (scrapping all work invested in these environments), and one should be very aware of that, prepared to start over, which becomes much more difficult/expensive, the more has been invested/trapped into such offers. Still fine for tools that help with a job/task, for which there also are similar (ideally compatible) alternatives available. Sometimes, gratis or no-dev/-code or gratis hosting by a third party etc. can be way too expensive (hidden costs, technical debt, opportunity costs + friction, rescue/liberation/migration costs, always to be paid by somebody, later). Just a warning, if one too quickly jumps into these 
Can’t view it, I’m not on Twitter any more. Export available?