According to data from Twitter compiled by eMarketer, 600 advertisers have partnered with the social network as of June 2011. That's up from 150 advertisers since the social network rolled out its Promoted Products in April 2010, suggesting many brands are eager to join the fold. "With a growing ad platform such as Twitter, a willingness to experiment and be flexible is essential for marketers," said eMarketer analyst Kimberly Maul. "Twitter advertising is a new and constantly changing arena, one that requires a mix of focused strategy and flexibility." A complete report, "Twitter Advertising: Four Marketers Test the New Ad Platform," is available to eMarketer Total Access subscribers. To learn more, click here.