Wikis are the most prominent example of online communities of collaborative content creation, of which Wikipedia is the largest and most famous. The primacy of research on Wikipedia in the literature has eclipsed the study of the wiki ecosystem, which actually displays a wide variety of demographics, usage patterns, governance, content and population dynamics. Many wikis struggle to survive, competing against one another for quality and contributors, growing in content and population or, conversely, threatened by inactivity or vandalism. This paper is the first empirical assessment of a large sample of wikis, their structure and their evolution over time. We analyse the correlation of various macroscopic indicators, structural features and governance policies of wikis with specific growth patterns, and discuss possible future research on the viability of online communities.