Except Zimmerman was well within his rights to figure out where the dude that just ran away from him went, as that's apparently what he felt the dispatcher was asking of him. There is nothing to say he explicitly followed Martin, and I would not consider watching from a distance and trying to see where the guy went "following".
It is suggested repeatedly, however, that Martin was just on his way back home. How, then, did the two meet if Martin had a head start that would have put him home twice? Even if Zimmerman was trying to follow him, he still did not know where Martin was for at least two minutes after he ran. This points to some obvious questions about why Martin was still out and about and in the vicinity of Zimmerman when he had so much time to get home or even out of the general area if he was so scared of Zimmerman.
JMO