Social networks, such as Twitter, are often seen as representations of social interactions, but they do not accurately reflect actual interactions. People tend to interact with a small subset of their declared contacts, as attention is a scarce resource. A study of Twitter data reveals that the true drivers of user activity are sparse, hidden networks of direct interactions, not the declared list of friends and followers. The research shows that users who communicate directly with others through direct messages have a much smaller number of actual friends compared to the number of people they declare as followers. This indicates that the network of actual friends is sparser and more influential than the declared network. The number of posts by users is more closely related to the number of friends they have rather than the number of followers. This suggests that the real social network that matters is the one composed of direct interactions, not the network of declared contacts. The study also shows that the number of friends a user has tends to saturate as the number of followees increases, indicating that maintaining direct communication is more costly than simply following someone. These findings highlight the importance of identifying the hidden, actual social network that drives user behavior in online platforms.Social networks, such as Twitter, are often seen as representations of social interactions, but they do not accurately reflect actual interactions. People tend to interact with a small subset of their declared contacts, as attention is a scarce resource. A study of Twitter data reveals that the true drivers of user activity are sparse, hidden networks of direct interactions, not the declared list of friends and followers. The research shows that users who communicate directly with others through direct messages have a much smaller number of actual friends compared to the number of people they declare as followers. This indicates that the network of actual friends is sparser and more influential than the declared network. The number of posts by users is more closely related to the number of friends they have rather than the number of followers. This suggests that the real social network that matters is the one composed of direct interactions, not the network of declared contacts. The study also shows that the number of friends a user has tends to saturate as the number of followees increases, indicating that maintaining direct communication is more costly than simply following someone. These findings highlight the importance of identifying the hidden, actual social network that drives user behavior in online platforms.