Si te he visto no me acuerdo

[ROBINDESBOIS]

EJEMPLO:
Lo que no me gusta de los X es que vienen a tu casa, pasan una semana contigo, son adorables, luego cuando vuelven a sus países, si te he visto no me acuerdo. (Meaning : You never hear from them again )

[Rusty]

This has been asked before (http://forums.tomisimo.org/showthread.php?t=4644).

Never to be heard from again is a good equivalent.
These folks could be classified as fair-weather friends. They're in your life for awhile, but usually only when it's convenient for them.
Here today, gone tomorrow is the only expression I can think of that describes something that is temporary. It can be applied to several situations, including friends that never were.