In late 2009, PayPal president John Donahoe indicated that he believed online payments should account for 20% of global payments, even though, at the time, they accounted for just 5%. His goal: find ways to grow that number.

There are a lot of ways of doing that, but none may be as promising as mobile payments.

As reported by VentureBeat, PayPal handled $4bn worth of mobile payments in 2011. While that number may not appear to be a lot in a global payments market that sees trillions of dollars worth of transactions every year, it does represent exponential growth, as PayPal did just $750m in mobile payments in all of 2010, and only $141m in 2009.

That kind of growth has proven more rapid than even PayPal’s own expectations. PayPal’s mobile VP David Marcus told VentureBeat that the company expected $1.5bn in mobile payments last year, but handily surpassed that thanks to a general growth in mobile commerce, the rise of the iPad, and specific applications, such as the Starbucks app which lets coffee drinkers buy credits that can be used in-store.

The question now for PayPal is just how well it can maintain its momentum. Clearly, it is off to a strong start in the mobile payments space. which isn’t surprising given its position in the online payments space. But given the promise of mobile commerce, it will face plenty of competition. Credit card companies, like Visa, are hoping to play a big role in the market with NFC technology, and upstarts like Square are looking to be a disruptive force.

One thing is for sure: the mobile payments space is going to be lucrative, and no one company, PayPal included, is going to win it without a huge fight. That’s good news for consumers and merchants looking for options when they use their phones to make a payment.