The Creative Group recently reported that 40 percent of companies using social media marketing believe assessing the channel's contribution to their web marketing success is their No. 1 challenge. While there are other concerns, many companies using Facebook , Twitter and other social networks have enough experience in the channel to avoid those associated with new adopters. Companies are seeing their campaigns attract frequent engagement, but they're not entirely certain of its effect on conversions and lead generation. According to the Creative Group, the dynamic nature of social media is among the main reasons they struggle to measure ROI effectively. Other concerns cited in the study include staffing social…