Don't be antagonistic. Lead with what the player wants, not with what is more viral. For example, when prompting the user to select friends for any kind of request, always suggest friends that are already playing the game. It is least likely that the player is interested in sending requests to non-players.
No direct solicitation. Don't task the user with spending their currency. Instead of tasking them with purchasing a specific item, task them with achieving something that may require them to purchase those specific items. Keep it goal-oriented, let the player find the solution.
Every click should do something interesting. Prefer click-do to click-select-do. The player will only achieve click-zen if each click is meaningful.
Social games need meaningful interaction between players. When I accept someone as a neighobr I'm agreeing to play a game with them, and they are agreeing to play a game with me. I should be able to take actions that cause meaningful changes to my neighbor's game state. And they should be able to meaningfully affect my game state.
It's the player's game, everyone else is just invited. I should be able to decline or undo, without cost to me, any actions from my neighbors. I don't have to explicitly confirm each action, but I have to be able to revert the effects of any action. This prevents griefing while instilling a sense of ownership over one's game world. Treat it like a blog: guests can comment, owners can moderate.