How big is the Bitcoin network? How big is the Bitcoin network in terms of total number of computers (peers, clients) that are connected together? Are there any network statistics or historical size data available?
My current real-time estimate using https://github.com/ayeowch/bitnodes is 8000 to 10000 nodes that are reachable at any one time with churn rate of 300 to 500 nodes every 5 minutes. The churn could be due to several factors, e.g.


Node went offline
Node reached max. allowed connections
Time of day, e.g. more nodes are online during US working hours
Network errors