Pick your three favorite movie characters for which there is a lot of information about them (name, town where they grew up, age, dog with a name, etc.). Rotate through these three. Append the name of the service. Dog's name? buddylastpass
There will be no reuse, because for Facebook it would be buddyfacebook or dugfacebook, or something else… but you will always be able to guess it in three tries. A computer system doing some kind of pentest isn't going to parse out the "facebook" or "lastpass". A human might, but that's why you rotate through three names. At the point where you have a human targeting your account and actually thinking about your inputs you are probably !@#$ed anyway.
There will be no reuse, because for Facebook it would be buddyfacebook or dugfacebook, or something else… but you will always be able to guess it in three tries. A computer system doing some kind of pentest isn't going to parse out the "facebook" or "lastpass". A human might, but that's why you rotate through three names. At the point where you have a human targeting your account and actually thinking about your inputs you are probably !@#$ed anyway.