Ever heard of the Network Effect? Your activist group uses insta to communicate with the public because everyone is on insta, so you don't want to move because of your followers who don't want to move because of you.
One work around is to mirror your posts to fedi. Post the same adverts and so forth in both places. This has specific utility when communicating in Signal groups.
Let's say everyone in your signal group is using their code name. You know the 'legal' names of your friends, but not everyone in the group, which tbh, might be compromised. Nobody wants to be doxxed.
Ok, so Alice makes a post about an upcoming picket and Bob posts it to a signal group and forgets to remove the tracking section of the URL, I click on the link and it says 'Bob shared this with you!' I now know that Blackbird also goes by Bob, as does everyone else in the group. Having a fedi copy is a URL that can be shared around Signal without leaking this kind of data.
Yes, its possible to remove the tracking portion of the URL, but this is an extra step that will be often forgotten even by experienced people. It is specifically designed to make this kind of opsec fail.
But there's more....