Analysis of content streams gathered from social networking sites such as Twitter has several applications ranging from content search and recommendation, news detection to business analytics. However, processing large amounts of data generated on these sites in real-time poses a difficult challenge. To cope with the data deluge, analytics companies and researchers are increasingly resorting to sampling. In this article, we investigate the crucial question of how to sample content streams generated by users in online social networks. The traditional method is to randomly sample all the data. For example, most studies using Twitter data today rely on the 1\% and 10\% randomly sampled streams of tweets that are provided by Twitter. In this paper, we analyze a different sampling methodology, one where content is gathered only from a relatively small sample (<1\%) of the user population, namely, the expert users. Over the duration of a month, we gathered tweets from over 500,000 Twitter users who are identified as experts on a diverse set of topics, and compared the resulting expert sampled tweets with the 1\% randomly sampled tweets provided publicly by Twitter. We compared the sampled datasets along several dimensions, including the popularity, topical diversity, trustworthiness, and timeliness of the information contained within them, and on the sentiment/opinion expressed on specific topics. Our analysis reveals several important differences in data obtained through the different sampling methodologies, which have serious implications for applications such as topical search, trustworthy content recommendations, breaking news detection, and opinion mining.