Facebook games (and MMOs in general, for the most part) are designed tightly around a feedback loop meant to make the game as addictive as possible. It strongly reminds me of experiments where rats are given a choice between eating and receiving a shock of pleasure:

http://en.wikipedia.org/wiki/Pleasure_center#Experiments_on_...

The worst part of these games is that they can act as a surrogate for success: your life may not be progressing in any way whatsoever, you may have goals and aspirations that you are not pursuing, but you are getting rewards and constantly progressing in a game - I'm not sure how good our brain is at distinguishing between real and virtual rewards (especially when the virtual rewards are guaranteed, but the real rewards are... well... hard).

http://en.wikipedia.org/wiki/Reward_system

All this is to say: other potential time wasters (TV is probably the most common) may be less likely to cause addictive and/or harmful behavior (I can't believe I'm arguing that TV may be less harmful than video games, but there it is). Sure, the majority of people can play causal games strictly in their downtime, but for some it spills over into what may otherwise be productive time, and for a relative few it can take over (where do you think Zynga's revenue is really coming from?)