After asking a few native Spanish speakers, the common phrase that equivocates to the English "Exactly!" is "¡Exacto!" (in the sense of "That's right!", that is).
I've heard the same people use both phrases interchangeably.
But literally translated, I believe exacto = exact, while exactamente = exactly
Can exactly be also Efectivamente? I have heard it lot among spanish natives.
It's like the "-ly" in English words. Surely, exactly, definitely, completely, etc.