FMyLife's new site accommodates account removal beautifully. Under "Profile" (http://www.fmylife.com/account/infos) simply click the button at the bottom of the page: "Request that your account be deleted."
You will receive an email and follow its instructions to confirm the action, then you will be logged out and your account is immediately erased.
Don't forget to first review what content and comments you've submitted in the past though, as it's unclear if that is also removed.
If you signed up for FMyLife using a third-party login such as Facebook or Twitter, you probably want to revoke all access from FMyLife. Click the icon below to find out how.