I built something like this for a hackday (actually it's still up http://worm.cloudfoundry.com/) - I only ran it with 50 people so I can't really speak about the scale, but I one thing I found was that lag can be a significant issue.
Different people will have different lags so you need to take that into account and adjust the historical view appropriately (i.e you can't just chuck all incoming data into a "current" bucket).
I built something like this for a hackday (actually it's still up http://worm.cloudfoundry.com/) - I only ran it with 50 people so I can't really speak about the scale, but I one thing I found was that lag can be a significant issue.
Different people will have different lags so you need to take that into account and adjust the historical view appropriately (i.e you can't just chuck all incoming data into a "current" bucket).