There's several things going on here.
Will you lose your Facebook info? That depends on what you mean. If you delete your account from within the, then yes it will delete it everywhere. If you go into the application manager and delete the app data (as I'm assuming you are referring to), then it just reverts the app to a freshly installed state. When you log in again, all of your account info will still be there.
Cleaning apps: Just delete it. These apps generally cause more problems than they fix because they fight how Android is designed to work.
Running out of memory: Again, we need clarification. Memory is typically referring to your RAM, which is what those cleaner apps are usually checking. Android works best when RAM is full, so that cleaner app telling you it's low is based on incorrect information. Android is not like Windows. A lot of people also now refer to internal storage as memory, which leads to confusion. If you are running out of storage, then that is something worth addressing. If you create an account to reply and let us know which you're referring to, we may be able to offer better advice.