Social Engineering - Exploiting the weakest link in the Security Chain

This article in Security Focus gives a vivid overview of Social Engineering and makes the point that Humans are the weakest link in the Security chain. What if we have a technically very strong secure system and give away the key to a stranger ? The system sure becomes vulnerable.
In real world, Intruders could collect information from us which we think of as trivial (such as phone number or address etc.)and can connect the dots and could use them to exploit us. Beware of your Society !!