Project user data

I'm sure this is something simple that I'm overlooking. I really don't know much about Excel except what I've been forced to learn on the job.

I have sets of data with three kinds of data:
Column A is years
Column B is number of persons using a specific service.
Column C is number of persons within the total population set

I need to project out to 2015

Currently I'm using the Forecast function like so:
=FORECAST(2011,B3:B6,A3:A6)

The problem is that the data results that I'm getting don't seem to jive with expectations.. they seem a bit too inflated.

What would be a better function to use in order to project the change in the users and population based on the past trends?

Thanks so much for any help!

PS - This is my first post.. I spent a lot of time searching and could not find anything that made sense for my application so I apologize if I missed something.

PPS - I attached a file to show what I'm talking about. Sorry I didn't do it sooner.