The ability for a political campaign to track down users on their personal devices is something not everybody is happy about, as ProPublica pointed out last week. Pandora mobile app user Crystal Harris called it harassment when a pop-up ad for Mitt Romney interrupted her listening experience on her iPhone. "To help Mitt Romney become the next president, Romney for President Inc. would like to use your email address — tap OK to let Pandora share this info," read the message, known as a tap-to-register ad.

Harris later announced on her Twitter feed that she responded by deleting her Pandora account and started buying songs off iTunes. 

But judging by Pandora, mobile political advertising is only going to increase from here. Pandora first started offering ad space to political users during the 2010 midterms and has grown from there, Duggan said. 

"The only thing that’s changed since 2010 is our scale," he added. 

Pandora offers a range of ads, including videos that play when users change stations, display ads and tap-to-register ads like the one that offended Harris. The tap-to-register ads interrupt the listening experience in the same way that any other ad would, and do not require the user to interact with them. If the user is interested in more information, ads like the Romney one described require a "triple opt-in" process, according to Pandora. The user needs to click on the ad, click "yes" when asked about the mailing list, and then accept or decline a subsequent email from the campaign. Google and LivingSocial have also used this form of ad.

Ad buyers are eyeing popular, social-based sites like Pandora and Facebook due to their ability to reach a huge, targeted audience. It is a more effective means of reaching the right audience, and one reason it is so effective is because the majority of Americans are mobile now. Research firm eMarketer found that by the end of the year, nearly 82 million Americans will access a social media site on their phone at least monthly. According to Pandora, more than 70 percent of their listening happens on mobile or "off the computer" (in July, total listening hours hit 1.12 billion, according to the company) and Facebook said last month that 543 million, more than half its users, access Facebook from a mobile device. 

Romney's campaign also recently used Facebook mobile ads to promote its "Mitt's VP" app ahead of the candidate's running mate announcement last weekend, targeting iPhone and Android users for their respective applications.