Also go into your facebook settings and go to "Aps and Websites" and make sure you remove any old apps and whatnot from there.
I work in IT security, let me tell you, it is EXTREMELY common for apps you install on your phone, as well as anything you integrate with facebook or google, to take WAY FUCKING MORE INFORMATION than they actually need from you.
With apps you can at least lock this down permission by permission, but when you integrate something with facebook, often times they get access to TONS of your personal data that they don't need, and you do NOT have granular control over it.
For example, an app might be able to get your locations you posted statuses from, or your relationship status, your cell phone number, and more. It might not need ANY of those things, but they take it. Why? Because they can monetize it. When you blindly agree to their privacy policy (we all do it, I do it too. I am not reading all that legal bullshit), you often agree to let them sell your info to third party organizations, or at least to use that info to allow for more targeted advertisements.
This is a GREAT article about facebook. The section "see who's snooping in your account" will be of interest from you. You can see the location sand devices where you've logged in from. It's possible that your ex has hacked into your profile and is able to get in and stalk you that way.
15 Hidden Facebook Features Only Power Users Know