AdultFriendFinder was hacked (again) in October 2016. According to LeakedSource, which acquired a copy of the dataset, this amounts to more than 400m accounts, many with plaintext passwords, from AdultFriendFinder and associated websites.
The site was compromised with a local file inclusion exploit, which means the website's code allowed access to files on the server that aren't supposed to be public.
Nearly a million accounts have the password "123456". More than 100,000 have the password "password".
The non-plaintext passwords were easily cracked anyway, apparently due to some roll-your-own encryption that involved lowercasing everything, SHA1ing it and going back to bed. The longest passwords were "pussy.passwordLimitExceeded:07/1" and "gladiatoreetjaimelesexetjaimefum", with a Blackadder fan in #3 with "antidisestablishmentarianism" and a sybarite who reads XKCD in #4 with "pussypussymoneymoneyweedweed."
Hotmail was the most common email provider, followed by Yahoo and gmail. These three accounted for the vast majority of registered addresses, with AOL and Live an order of magnitude down.
Leaked Source isn't making the data set publicly available; but if they have it, others might too.