This is some interesting data from the Chicago tribune on the Divvy system between opening and August 15 (sure it's a little out of date but I didn't see it posted here). It includes a breakdown of trips by station - the most popular stations in the week they examined was Streeter Street and Illinois (Navy Pier). Other stations along the lakeside path were also among the most popular.
Average number of trips in the latest weeks is about 4000-5000 (24 hour + annual members) for weekdays and 6000-7000 for weekends.
http://www.chicagotribune.com/media/...0-18174940.pdf