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Sizing up online social networks

While the size of popular online social networks such as MySpace and Twitter has been reported to be in the tens or hundreds of millions of users (and growing), little is known about the fraction of users who have either deleted or abandoned their accounts. Therefore, the growth of an OSN's overall user population and, more important, its population of active users cannot easily be determined. In this article we describe a measurement technique to infer the fine-grained growth in the total number of allocated accounts for a class of OSNs that include MySpace and Twitter and are characterized by two features. First, they assign numerical user IDs using a format and allocation strategy that can be determined. Second, a fraction of their users have abandoned these OSNs shortly after creating their accounts, and these short-lived users (called 'tourists') are scattered across the ID space. By exploiting these two properties, we are able to determine the growth in total and valid user accounts for MySpace and Twitter since their inception. For valid user accounts, we also derive the fraction of active users in the system at the time of our experiment, where we define the activity of a user in terms of the recency of her last visit to the OSN. In the case of MySpace and Twitter, our results show that the active population of these OSNs is typically an order of magnitude smaller than the reported (total) population.