Plenty of fish stores your password as plain text in their database. I know this because they send you a weekly email with updates to the site and puts your password at the bottom of the email so you can easily login.
Plenty of fish was hacked and their data was stolen by a 3rd party. They now have your email address and a password to try for other sites.
Do you use the same password for Plenty of fish as your Facebook? Yes? Well someone can now login to your Facebook account and pretend to be you. maybe change the password and tell all your friends that you actually like having sex with farm animals or your cheating on your wife with so and so.
Well what am I suppose to do, I can’t remember 1000s different passwords?
Well you could use a program to generate and store the passwords for you. Something like Keepass. It works for all phones and computers, anything that can run on java. You could hash the domain name of the site and use it to generate a password for you.
Here are some tips for creating new passwords.