Twitter has 130,000 active advertisers

"In Q4, we reached 130,000 active advertisers, up almost 90% year over year, driven by our small and medium-sized businesses (SMB) initiatives. We expect that SMB growth will continue as we improve our product, making it faster and easier to run campaigns and improve our direct response tools. "
Source:  Twitter's Q4 2015 Results statement, 10th February 2016