Part of the problem is that people rarely make a conscious decision to pull the plug on a site or service. You might just stop visiting a site frequently, and eventually forget about it entirely. It takes a little work to stay on top of these things, but you should make the effort to ensure that you don't expose yourself to undue risk or leave sensitive information vulnerable.
Use a password-management utility
It isn't easy to come up with unique passwords, never mind keeping track of all of them. A 2012 survey found that most adults have five or more unique passwords, and that nearly 10% report having 20 or more passwords. Major data breaches over the past few years, however, have exposed the fact that many of those passwords are easily guessed strings (like "12345" and "password") that provide essentially no security at all.
Kandek learned his lesson after the Stratfor incident. It prompted him to change his behavior and start using a password manager to generate single-use passwords rather than reusing the same password over and over. "I have been very disciplined, and it has proven quite workable and useful. I use LastPass because they support Linux and Chromebooks well and offer two-factor authentication."
Of course, an online service like LastPass is itself a risk, so it's not exactly a silver bullet. There was some concern in 2011 that LastPass may have been breached, but that turned out to be an overreaction to anomalous network traffic.
Nevertheless, be sure to follow these tips and take steps to deactivate or delete unused services and applications, or your zombie accounts will eventually come back to haunt you.