This holiday shopping season, retailers are witnessing first-hand just how important mobile has become to their businesses.

And they’re not alone. According to Nielsen’s Social Media Report 2012, mobile is now crucial to social media as it is driving the growth of the most popular social networks.

In total, U.S. consumers spent 121bn minutes on social media sites in July of this year, a surge of 37% year-over-year. And the majority of that was driven by mobile, as consumers increased their usage of social mobile apps and websites by 63% this year.

In looking at the most popular services, Nielsen found that the number of unique users of Facebook’s mobile app grew 88% year-over-year, while the number of unique mobile visitors to the Facebook website jumped 85%. Twitter experienced even more staggering growth: its mobile app audience rose by 134% and its mobile website audience leaped 140%.

Given that Facebook’s total U.S. audience declined 4% in 2012, and Twitter’s grew by a modest 13%, it’s clear that the continued success of the most popular social networks will depend heavily on how well companies can create compelling mobile experiences.

It will also depend, obviously, on how rapidly they can develop ways to effectively monetize those experiences through advertising solutions that drive sales for marketers. On this front, Nielsen’s data should raise concerns.

While the analytics firm found that just over a quarter of consumers are “more likely to pay attention to an ad that has been posted by one of their social network acquaintances,” a third of users find social networking ads to be “more annoying than other online ads.” Just a quarter of users feel comfortable with ad targeting based on profile information, and well under a fifth (17%) “feel more connected to brands seen on social networking sites.” All told, only 10% of social network users have made an online purchase based on a social ad, and just 8% have made an offline purchase.

While marketers continue to invest heavily in social, and Facebook and Twitter have been able to roll out ad offerings suited to their vast mobile audiences, the figures published by Nielsen serve as a reminder: the existence of mobile ad products, and marketer adoption of them, doesn’t mean that social nets and marketers are any closer to solving the mobile monetization challenge than they were at the beginning of the year.