i play the odd face book game, currently enjoying "my tribe". We might need to know more about the ideas you have before we can shoot specific ideas at you but the current equation for these games is that they take time and lots of it. spending money either makes it take less time or allows you to be way more efficient with the same amount of time.
examples: farmville, mafia wars, hero wars, my tribe, cafe world
another thing that "makes cash" in these games is cash only items. you can buy plenty of things with in game money but some/many things are cash only. They might be purely aesthetic or give some nice big bonus.
examples: farmville, gunbound (not facebook but notorious for this)
thirdly, and I really think this goes without saying, facebook games have values and stats that you use to compare yourself with others. why have a game on a social network if it doesn't keep track of all your work and displays it for you in some way? This is why in most of these games you can "visit" your friends.
some things I would like to see:
- more incentive to work together without being limiting*
- A way to (slowly) gain "cash" points (not in game money)**
- rewarded for playing, not punished for "not playing"***
some things I would hate to see:
- PvP not optional****
- cash required to continue at some point*****
- cash only items/things, at least make them really expensive for non paying customers
- you need X number of friends to...
*: in Farmville you can visit other people's farms which is all fine and dandy. but what can you do? fertilize a few of their plants and do something with their chicken coop last i heard. you cant save their ready to wilt crops or help them harvest anything. why don't they allow people to be helpful?
**: my tribe, mafia wars, hero wars and I think farmville do this. you can buy like 5000 game cash for $10 and you can also get 100 in game cash when you level up. most of the sweet stuff costs enough in game cash that getting it for free takes a lot of effort (but at least you can!)
***: in farmville your crops wilt, in cafe world your food goes bad, in my tribe if you leave for long enough, your oldest tribe's person dies of old age and in egg breaker your hammers expire. these are all great hooks to keep people playing but they are not fun. rather its more fun to reward the player for being there and putting time into the game; in my tribe certain items appear on a rather scheduled basis which you can collect if you go looking for it. the game lets you know when they will appear but they wont if your not playing the game. this is actually the only way to get most of the items in my tribe.
****: I HATE THIS. maybe it fits the theme of the game well, maybe that's what your game is about and i should just not play it but... getting ganged up on by someone with 235 friends over and over, not being able to do anything because i never have any resources or because i don't have enough friends. it just isn't fun. PvP has the potential to be a lot of fun (just look at EVE online or pretty much any FPS with online play). mafia wars did this. I quit playing because any time i made any money it was instantly gone. people on facebook gang up big time and it just isn't fun.
*****: "and now I have built everything that doesn't require my pocket money. There is more content that i could enjoy but i would have to pay for it. it probably isn't much different from what i have already done so i wont buy into it." This was pretty much my problem with runescape. runescape had interesting and amusing quests, interesting areas to explore and treasures to find. the crafting system was addictive and enjoyable and took real dedication to get far in. but after a bit of exploring you come to a great wall with a gate in it and on the gate it says "pay us money and you can enjoy the other side of this gate!". Also, as a hard working crafter, I was quite pissed when I couldn't craft half the things available to me because i wasn't a paying subscriber. to top it off, and this made me quit, the new stuff was only for subscribers, nothing for the rest of us.
sorry for the length of the post. I would really love to read about what kind of game your thinking of ^.^