Livejournal was hacked in 2014. Someone managed to download a list of all users, and has posted it on the web. (I'm not telling exactly where, for reasons that will become clear.) It contains over 30 million records with email addresses, usernames, and raw passwords.
You know what to do when your password is leaked: You change it to something new, something you haven't used anywhere else, something complicated. Maybe you store it in a password manager so you don't have to remember it. And check to see if you used it anywhere else, especially on Dreamwidth.
But this is worse: Your email address is now linked to the usernames of any accounts you created with that email. Here are some scenarios:
Ah. LJ is (98% certainly) the 'another social media service' that isn't admitting it. Cool. Cool. I used different email addresses for each of my LJs, but I think they crossed over (like, the address for my fandom lj wasn't thatusername at gmail, it was otherusername and so on in in criss-cross).
H/T