We have identified an issue with the display of invite virality tracking in the dashboard that currently affects related metrics from June 9, 2010 onwards.  We are working to resolve this issue and retroactively correct the display of these metrics as rapidly as possible.  No data has been lost.  Sorry for the inconvenience.  

SOLVED (June 15, 2010): 

Issue has been resolved and data for all accounts has been retroactively corrected.