Google officials in India earlier this year were alarmed to learn that Facebook was likely to generate about $980 million in revenue in the country in 2018, according to one of the sources. Google's India revenues reached $1 billion only last year.

Facebook's success has shaken Alphabet's Google, led by an Indian-born CEO, Sundar Pichai , who has made developing markets a priority.

Google retains only a slight lead over Facebook in the competition for digital ad dollars in the crucial India market, sources familiar with the figures say, even though the search giant has been in the country far longer and has avoided the controversies that have dogged its rival.

Sundar Pichai, chief executive officer of Google Inc., attends a news conference in New Delhi, India, on Wednesday, Jan. 4, 2017.

Facebook and Google declined to comment on Indian revenue figures or the competition between the two companies.

Google is now pushing back, attempting to lure customers with better ad-buying tools and more localized services. The revamped strategy mirrors initiatives that have succeeded in boosting the time Indian consumers spend with Google services.

The battle in India reflects an epic challenge for Google in developing markets around the world that are crucial to the company's long-term growth — many consumers in those countries are gravitating to Facebook and it's siblings, Instagram and WhatsApp, at the expense of Google search and YouTube, and advertising dollars are quick to follow.

"Facebook is a far more user-friendly platform even though they haven't created features specifically for Indian advertisers," said Vikas Chawla, who runs a small ad-buying agency in India.

Facebook ads, compared with those on Google search or YouTube, tend to transcend language barriers more easily because they rely more on visual elements, said Narayan Murthy Ivaturi, vice president at FreakOut, a Singapore-headquartered digital marketing firm. Pinpointing younger consumers and rural populations is easier with Facebook and its Instagram app, he and other ad buyers said.

And Facebook is succeeding in India, which boasts the fastest-growing digital ad market of any major economy, despite internal turmoil and political controversy. It has been without a country head for the last year, and has faced a series of incidents in which rumors circulating on Facebook and WhatsApp have prompted mob violence.

Facebook and Google between them took 68 percent of India's digital ad market last year, according to advertising buyer Magna. Media agency GroupM estimates digital advertising spending will grow 30 percent in India this year.

The Facebook phenomenon is evident close to home for Google.

During a recent lunch period, six out of 10 people who walked out of Google's Bangalore offices while looking at their phones told Reuters they were checking WhatsApp. All 10 said they regularly used Whatsapp.

Eight Indian ad buyers interviewed by Reuters were divided on whether Facebook would overtake Google in Indian ad revenue. That such a question would even be debated explains why Pichai, Google's chief executive, has pressed to flip the company's approach to emerging markets.

"India is the most important market for the 'Next Billion Users' initiative," Caesar Sengupta, the head of the effort, told Reuters on the sidelines of the annual "Google for India" event in New Delhi last week.