@maxprime You can't log into other Fediverse services using your Lemmy account. It's a Lemmy account for your specific Lemmy instance and doesn't work anywhere else.
What you can do is talk to people on other Fediverse services with your Lemmy account on your instance in the same way you are communicating with people from other Lemmy instances.
For example, the op you are replying to is using Friendica, you are using Lemmy and I am using Firefish right now.
Well looks like a Lemmy comment is making the rounds on Mastodon! (nerdica.net) en-gb
nerdica.net/photos/masimatutu/…...