While advertising revenues have been disappointingly low for most applications on Facebook and other social networks, another option app developers are increasingly turning towards is micropayments for virtual goods or premium features. Both Facebook and MySpace have admitted that they are working on their own payment systems, and Apple could play a role as well since it already has a payment system in place for iPhone apps (although even Apple is running into some bumps).

While the bigger players are fiddling with their payment system plans, nimbler startups are moving in to fill the gap. One of these is Spare Change Payments, which is trying to become the Paypal of micropayments. A year after launch, more than 700 apps across Facebook, MySpace, and Bebo use Spare Change for micropayments. Spare Change is processing $2.5 million a month in micropayments, which is a $30 million annual run-rate. The apps that are having the most success with micropayments are games and ones that sell virtual goods.

Now, the company is making it easier for consumers to pay through Spare Change with a new payment widget that pops up in each app instead of sending people off to a separate payments page.

Global mobile payments network Paymo and social network hi5 have partnered to let members in 24 countries use their mobile phones to purchase hi5 Coins, hi5’s virtual currency. Countries where Paymo will be made available to hi5 users include the U.S., Canada, France, Hong Kong, Thailand, Russia and Colombia.

Paymo’s mobile payment system doesn’t require users to have a credit card or bank account. Users enter their cell phone number on the site, reply to a Paymo text message and then all virtual charges on hi5 are automatically charged to the user’s monthly cell phone bill. It’s pretty easy and similar to UK mobile payments service, Zong. Paymo says that over 75 percent of the online community worldwide does not have a credit card but 3 billion consumers own a mobile phone. With that in mind, the company is hoping their system, which avoids credit cards or bank accounts, will take off.

Users are increasingly choosing dead simple SMS mobile payments for micro-transactions on social network applications and gaming sites (it fills the void while they wait for more direct options), but super-high transaction fees are limiting growth.

The problem is that legacy transactions - specifically scams that give users a “free” ring tone with the fine print mentioning a monthly charge as high as $20 - have brought in so much cash to the carriers that they’ve gotten used to taking 50% or more of the total payment in fees. For the market to grow to encompass legitimate transactions, those fees have to drop dramatically. For that to happen, the social networks need to get involved directly in carrier negotiations.

Two companies, both headquartered in Europe, are already targeting mobile payments for apps - Mobillcash (UK) and Zong (Switzerland).

When you buy a virtual shotgun on Mobwars, for example (and they are selling a lot of them, up to $1 million per month) you have to pay real cash. You can choose to pay via a number of services (Facebook doesn’t offer a direct payment solution yet), including either Mobillcash or Zong.

Mobile payments startup Zong, which is one of our DemoPit companies at TechCrunch50, thinks it has the answer to micro-payments on the Web for social apps like gaming, dating and classifieds. It uses your mobile phone as a way to pay for things such as virtual goods in Facebook apps.

The way it works is that you enter your mobile phone number to pay for something on the Web, maybe a virtual cowboy hat for 5 cents. Then you get a test message on your phone with a pin number. If you enter that pin number in a widget on the Web, the charge will be reflected in your cell phone bill.