Rhetoric about the “post-PC age” has ramped up considerably since Apple first launched the iPad in the beginning of 2010, adding a hot-selling tablet device to the smartphone-fueled wireless Internet revolution already underway.

PC sales have already started to wane, but it won’t be until 2015 that they’ll really take a hit. That’s the year that mobile Internet users will outnumber people accessing the Internet from PCs and other wireline devices, according to new information from International Data Corporation (IDC).

“The impact of smartphone and, especially, media tablet adoption will be so great that the number of users accessing the Internet through PCs will first stagnate and then slowly decline,” reads a statement released by IDC.

By that point, the total number of Internet users worldwide is expected to grow to 2.7 billion, which represents about 40% of the world’s population.

This isn’t the first time this type of prediction has been made, but it lends credence to the significance of tablet computers specifically. For years, we’ve heard about the number of smartphones outselling PCs worldwide, a prediction that became reality at the end of last year, when manufacturers shipped 100.9 million smartphones, compared to 92 million PCs sold.

While the revolution in computing spurred by smartphones is significant, it’s worth noting that the tablet computer only started picking up steam with consumers last year. The device that inspired that was, of course, Apple’s iPad, which launched in January 2010 and has seen explosive growth since, making up 21% of the company’s revenue in the third quarter of this year. Apple is expected to remain dominant in the tablet space for years to come, even as competition from devices running Android heats up and Amazon finalizes plans to launch its own Android-powered tablet at a significantly lower price.

The impact the tablet has had on the growth of the wireless Web is huge, considering how young the market for those devices is. Coupled with smartphones (not to mention smart TVs and a whole range of Internet-connected devices and objects yet to come), they’re paving the way for an age in which the personal computer is no longer dominant and computing shifts to handheld devices and a variety of other devices, objects and even household surfaces.

E-Commerce Spending to Exceed $1 Trillion

This latest survey also includes some numbers that will be of interest to Web marketers and media geeks. As more people connect to the Internet around the world, the amount of revenue generated by online advertising will grow by 61% from 2010 to 2015, IDC predicts. By 2015, online advertisers will make a total of $138 billion and the Web will increase its overall share of ad spending across all media to 17.8%

At the same time, e-commerce spending worldwide is expected to reach $1.2 trillion by 2015, a an 81% increase over 2010.