What are the limits of Buddhist compassion?

Compassion seems to be a central insight in Buddhist thought. When someone truly grasps Buddhist ideas they find themselves de-emphasizing their own ego, and understanding that they're interconnected with the world and everything in it. I've lived this way for a long time and have become very effective at helping others, serving others, cultivating meaning in the lives of others. However, lately I'm realizing that this compassion is often not reciprocated, and at worst is taken advantage of as others know they can count on me to help them. So I'm wondering if Buddhist thought has ever touched on this issue? How do we reconcile striving for unconditional compassion toward others, when many of those around us are happy to cheat, steal, and take from us?