Facebook has 2.5m advertisers

"In June of 2013, Facebook formally announced that it had 1 million advertisers. Then, in February of this year, the company announced 2 million advertisers. And last night, it said it now had 2.5 million.
Advertiser growth is clearly accelerating.
It would be interesting to know how many advertisers Google currently has. Financial analysts have estimated that Google has approximately 4 million advertisers today.
A substantial though undisclosed percentage of Facebook’s advertisers are small businesses. The company previously said it has 45 million active small business pages globally. It’s trying to covert more of those active Page owners into advertisers.
As part of the 2.5-million announcement, Facebook is introducing new products and tools directed at mobile marketers. The company reported in the second quarter that mobile advertising accounted for 76 percent of total ad revenues."
Source:  Marketingland, 28th September 2015