I'm curious to see how the site has grown over time and I check area51 periodically but is there any way to see how the area51 stats have changed over time?
This doesn't have all of the Area 51 statistics, but it does show the growth of the site over time with respect to traffic and statistics about the traffic: http://www.quantcast.com/salesforce.stackexchange.com.
I found the question How do I know if a beta Stack Exchange site is growing well? that has an accepted answer that points to Quantcast in the following snippet:
Stack Exchange uses Quantcast which collects traffic data about all SE sites and makes them publicly available. See the following URL for the data:
Just select the subdomain of the site you're interested in.