• by Gavin O'Malley  @mp_gavin, September 7, 2016

In yet another troubling sign for Twitter, the social giant’s global average CPM fell 18% to $4.29 over the past year, according to a new second-quarter ad index from Salesforce.

Worse yet, from the first quarter to the second quarter of this year, Twitter’s global CPM fell by a whopping 38%.

Facebook, by contrast, saw its global CPM rise 65% to $6.33, year-over-year. Stateside, the reigning social champion’s average CPM grew by 74%, year-over-year.

Facebook can also be proud of the fact that its prized Instagram unit saw its global CPM rise 42% to $6.30, year-over-year. That’s a big deal because about 80% of Instagram’s roughly 500 million users reside beyond U.S. borders.

Rounding out the top social networks, LinkedIn’s global CPM rose 13% to $29.43, year-over-year.

The ad data encompasses hundreds of millions of ad impressions and engagements by Saleforce’s Marketing Cloud customers who used it Advertising Studio platform from April through June of this year.

Despite its best efforts, Twitter continues to struggle. Recently, data emerged showing that ad agencies are increasingly bypassing the platform in favor other social networks -- Instagram especially.

That might explain why Twitter recently slashed its second-quarter revenue guidance to $590 million from $610 million -- nowhere near analyst expectations of $678 million.

eMarketer recently released a forecast suggesting that Snapchat will overtake Twitter in terms of domestic users, before the end of the year.