Ad measurement startup iSpot.tv announced this morning that it has raised $21.9 million in Series B funding.

The company says it knows exactly where and when national TV ads are running, and can therefore determine when those ads drive consumers to respond via Facebook, Twitter, YouTube and search. It has also created an online library of more than 100,000 ads.

Founder and CEO Sean Muller said brands and ad agencies often use iSpot.tv in conjunction with other analytics and attribution products, but he said, 'We focus on the areas we bring into the marketplace that no one else can provide - those two things are real-time data around TV advertising and then the digital performance data. How do our clients use this? For competitive intelligence in real-time, ad effectiveness in real-time, media planning in real-time.'

iSpot.tv has now raised more than $27.5 million in total funding. The new round was led by Insight Venture Partners, while investor Madrona Venture Group also participated.