In many social network games, most famously in Farmville, one of the core activities involves a mechanic that has come to be known as withering. This is actually a sub-mechanic, or one variation on an investing mechanic. In Farmville you can plant crops. These crops grow over the next 5 minutes or 2 days or almost any amount of time in between, depending on which type of crop you choose to plant. When the crop is fully grown you return to the game to harvest it. Crops cost a small amount of earned currency to plant and yield a much larger amount of earned currency when harvested. This basic investment mechanic is the major source of earned currency in Farmville.
This is a great mechanic. Players get to choose the interval of time the crop will take, based on their personal schedule. They are excited to return to the game later to reap the rewards. It provides a controlled rate of economic growth for the player that they have a hard time circumventing (so you as designer can predict accurately the maximum rate of income generation a player can achieve).
(I'll get to withering eventually, hold on.)
You might think that the longer something takes to grow, the greater the profitability it will have. Making the player wait longer for their money should mean they'll get a higher return, right? This is not how you want to design it. You want to encourage players to come back often so you make the 5-minute crop the most efficient. If players can spend all day playing your game you'll get more chances for them to spam their friends. Also, if you have an energy mechanic they will need more energy to harvest and plant so frequently - and since energy refills are a source of profit for you it will mean more potential income.
Withering occurs in some games that use crop-like mechanics. Once a crop is ready to harvest, it only remains ready for a fixed amount of time, typically twice as long as it took for it to grow. After that second inverval, the crop withers (dies) and cannot be harvested. You must click to clear it, and you lose your entire initial investment. This mechanic gets players to come back in a timely manner, making them feel guilty or like a failure if they forget. The avoidance of these negative emotions is a strong motivator.
This may be too punishing. Many players avoid these games entirely because they repeatedly lost their crops and were set back to a position worse than before they planted them. I believe that most of the effect can be gained without punishing the player as much. If, instead of dying completely, a withered crop could be revived with a short additional timer until it is ready to harvest (say another 5 or 10 minutes) players like me wouldn't leave the game.
Some games mitigate withering by allowing friends to un-wither crops for you. While getting this to work out can sometimes be awkward (like playing phone-tag), it provides a solution that uses the player's social network.
When working this sort of mechanic into a game you'll have to decide what the penalties are for players who miss the "harvesting" window. Can you make them feel they must come back on time without setting them back if they forget?