Unfortunately Diaspora does not use the ActivityPub protocol and doesn't even bridge to it. They have made the deliberate choice to not ever have a sizeable community.
It's been around for many years now and famously was used in the consultations for the constitution of Iceland. We also used it in Bulgaria back in 2013 and had a community of more than 3000 users, but it lost traction due to being ignored by politicians and controversial debates.
Can somebody also make join-lemmy.org notice that our instance exists? Can't get it listed, even though we comply with the requirements for a while now.
You could subscribe to the Lemmy community by following e.g. @fediverse@lemmy.world from mastodon. If you're familiar with https://a.gup.pe/ , it's exactly the same from mastodon. The difference is that with lemmy you can also access without passing through mastodon.
thank you!