Mobile payments system Boku just announced that it will work with a bevy of social networks and gaming sites. But competitor Zong, was recently chosen to pilot Facebook's virtual currency, called Credits. Both sites will face considerable opposition from Obopay, a seasoned startup that recently earned the backing of mobile phone giant Nokia for its Nokia Money payment system. Mobile payments are convenient, fast and easy--but which service should you use?

That depends on what you want to do. Mobile payments services work in two ways: some allow you to buy stuff from online retailers using your phone, while others allow you to send money to your friends. A couple of services do both. Here's the rundown.

If you're a worldwide user, it's hard to beat Boku's dead-simple setup and its 56-country availability. When you buy things online with Boku, you simply navigate to a participating site and punch in your cell phone number. You get a confirmation text, to which you reply "Y" to say yes to the charges. The sum then shows up on your cell phone bill. That's great for in-game purchases or retail items, but it doesn't let you send money to your peers.

Zong works similarly. The company got its start doing TV-based voting (think American Idol) in Europe, and then leveraged its carrier relationships to start the payments division. While Zong is only available in 19 countries, the company's been around for almost 10 years, which should be long enough to comfort any nervous adopters. Punch in your mobile number, confirm, and the charge appears on your bill. Again, this is only for paying for things online--not for spotting your friend a few bucks.

Obopay is more sophisticated, because it lets you actually send money to other individuals--not just online businesses. But that also means you need to create an account and charge it up with money, something that's not necessary on Zong or Boku. You can also use Obopay online, without a phone, and since it has partnered with Nokia, it'll have extensive worldwide availability before too long.

Not one to be left out, PayPal also has its own mobile payment system, and an iPhone app to boot. PayPal lets you send money either using a text message or its iPhone app, and in either case it will call you back after you enter your transaction so that you can confirm by entering your pin. Since PayPal is renowned for its aggressive security measures, there's no doubt that it is the most trustworthy of the bunch, but also the most subject to the hassles that come with high security. PayPal's fees are also among the more substantial, depending on how big an amount you're sending.

Amazon's Payments system also has a mobile iteration called Amazon TextPayMe, which allows Amazon members to pay (or request payment) using their Amazon accounts. Just like Zong and Boku, the whole process requires just a few text messages. The same system allows you to search and buy Amazon products with a feature called TextBuyIt, and like Obopay, there's a Web-based equivalent if you don't feel like messing with your phone to send someone money. You can also send someone a gift card using a text message, if you're of the ilk that likes to micromanage your recipient's spending. Overall Amazon's system is likely the best for most purposes: simple, secure and flexible, and with about as much simplicity as can be expected from something so capable. And chances are you have an Amazon account anyway.

One of the downsides of all these services is that you need to memorize text-message shorthand for each one. Most of the commands are simple, but with Amazon, for instance, things get a little complex when you're trying to request a payment from someone.

Impossible? No. But if you still haven't figured out that ALT + F4 closes windows on your PC, then you might want to stick with PayPal's user friendly iPhone app.