TL;DR: If you have or had a Livejournal account, treat the login details as compromised.
Which social network becomes popular with which bunch of people is more a matter of chance than anything else. Being a 'great' social network helps, but if another site is 'good enough' and has user base large enough, it doesn't matter. Until enough of them move elsewhere, that community will continue to use where they are.
One example is the way that Bebo – treated as something just for teenagers in the UK – ended up being the site for gay and bisexual men in Ireland to meet up, more popular for that than sites like Gaydar. Bebo didn't plan for that to happen, but until most of them moved to Grindr, it didn't matter: that's where they were.
Another example is that before Facebook existed, lots of people in the bi communities used Livejournal.com ('LJ') as their social network. I do mean lots: when someone ran a session at BiCon 2002 for people with LJ accounts, way more than half the attendees turned up to it! (Fortunately, the organisers suspected that would happen and put that session in a very big room…)
A variety of things meant that the number of bi people using LJ declined. Trying to find someone who still did use it was a task on the BiCon 2016 treasure hunt,* for example.
The rise of evilFB is one obvious reason for that, despite its problems, but the final straw for many people was when the site was sold to Russian owners in 2007. (LJ was, and still is, very popular there.) And after moving all the servers that power the site there, LiveJournal changed its terms of service to conform to Russian law… including the homophobic crap bits.
When the move happened, there were several people saying that you could assume that Russian intelligence had full access to the database, and it looks like they're not the only ones.
On the 25th September, bots started trying to email me on an address that had an LJ account. I don't know what the content of the emails were – the server blocked them early on in the process – but today, they found a bot that didn't get blocked.
The 'attempt at blackmail' email that it sent contains the Livejournal password for that email address.
That LJ account was deleted in September last year, and confirmed finally really deleted** twelve days before the emails started, but was the correct password for that account.
Given the password's length and the way I don't appear to have used the email address anywhere else – it doesn't come up in a Google search for it, for example, and there are no non-LJ emails to it – I can't think of any other source than the Livejournal site itself.
Fortunately, I've been using site-unique passwords since Hotmail was new and calling itself HoTMaiL, but because of how I generated those, it's password change time for lots of places… Fortunately (again), I've been using a decent password manager for a while too…
"Obviously", you should not reuse passwords from one account with another, especially on a different website.
But just in case you did, and you had a Livejournal account, change the password for wherever you reused it. Now.
If you did like LJ back in the day, do consider Dreamwidth. You can't post pictures of cats as easily as on FB, but its privacy is better than FB has ever managed, it's paid for via (optional) subscriptions rather than selling your info / advertising, and not least as it's based on the same code, you can easily import your old LJ to it.
* Organised by the same person who ran that session back in 2002!
** LJ's current owners don't actually delete things for a year, just in case you change your mind.