Advertising agencies allocated between 11 and 25 percent of their ad budgets to paid social media in second quarter 2015, a 24-percent increase from the previous quarter, reports media buying and selling software provider STRATA. Facebook is the leading choice among agencies, with 93 percent planning on using it in their campaigns, compared to Youtube (57 percent), Twitter (52 percent) and LinkedIn (29 percent).

The increase in social media ad spending has created some complications for agencies, reports STRATA. Its survey found that media mix comes in overwhelmingly as the biggest challenge facing 40 percent of agencies, surpassing client attraction, and marking an 85-percent increase from the same time last year. Following media mix was client attraction (24 percent of agencies) and client spending (11 percent). Similarly, 22 percent of agencies expect their clients to make minor budget cuts from last year.

“There’s an undeniable correlation between the rise in social media advertising with mobile device behavior,” says Joy Baer, president of STRATA. “Agencies and advertisers are going to follow their audience. Mobile users are checking Facebook and Twitter throughout the day. So when you consider that around 60 percent of digital media time spent in the U.S. is on smartphones and tablets, then it makes perfect sense to reach the audience in the apps that they’re already accessing.”

Click here for an infographic created by STRATA of their survey’s result.