Advertisers are increasing their spending on social networks faster than expected. Digital market research firm eMarketer’s September forecast points to global social network ad spending reaching $25.14 billion in 2015, up from the $23.68 billion the company predicted in April.

Facebook is the largest driver of the uptick; the social media giant is expected to capture $16.29 billion in advertising revenues worldwide in 2015, a 41.8-percent increase over its 2014 performance. In 2015 it will account for 64.8 percent of global social network ad spending. Its Instagram subsidiary is responsible for five percent of Facebook’s 2015 total as the app is slated to generate $600 million in 2015. Instagram’s ad revenues are forecast to grow 149 percent in 2016 to $1.48 billion.

eMarketer has throttled back on its forecast for Twitter. The social network is expected to grow ad revenues 61.8 percent in 2015, bringing in $2.03 billion. In April, eMarketer predicted 2015 growth of 66.9 percent. Debra Aho Williamson, eMarketer principal analyst, says, “Twitter's slowing user growth is impacting its ad business. Twitter has improved its ad targeting capabilities, and it still has a lock on real-time conversation. However, advertisers want to reach a mass audience and that's harder to do on Twitter than on Facebook.”

Read more on eMarketer’s expectation for social network advertising here.