Farmville
I can say that I enjoyed farmville just a very beginning moment when I started playing it.
Other than that, It's just like I have to keep doing the same thing every single day.
When I grew a seed, I need to come back again to harvest it otherwise it will become witter.
I don't like the feeling to make a schedule for myself to do the boring thing.

Friends helping system is doesn't make sense. For example, I needed a wooden board in order to
finish my beehive but I have no way to get it other than ask from friends(spend real money does not count).
But when my friends ask for a wooden board, how come I can send it to them by 1 simple click!!!
which is so ridiculous......

Cityville
This game got a huge improvement from farmville. I enjoyed playing this game for a couple of weeks before getting bored.
Basically, it has the same system as farmville. But what I must do is not just to plow a field, seed it and harvest it.
I can make a road, grow some veggies, build a house, run my business and many many quest to follow. Also when my town grow bigger,
I need to keep clicking on the coins and the stars to make combo and will grant me extra money(1500+ for full combo). This feature
make me a lot of fun everyday since started until now. Graphics of this game also a big leap
from farmville. Friends system has the same concept from farmville but little bit make more sense...personally


Conclusion
From both game, I think the only thing I hate for social network game is, I cannot do something just by myself.
I need to get more friends to play a game. I need friends' help to progress. This thing just ruin the fun factor for me.
But for most of people who is not a hardcore gamer, they may like these social network game.
A boy can be impressed just by getting helped from pretty girl. Employees can go to their boss' farm to make him happy and get a chance to be promoted in real life.
These sort of things definitely make social network game becomes popular.