I'm interested in understanding how fast a web service can expect to
grow - specifically free web services compared to a pay web service. 
Examples of a web service: email, virus protection, backup, file
format conversion, photo locker, etc.

I understand that this is a vague question - so what I'm looking for
are anecdotal evidences for website growth that are current (ie.
anything before 2003 is not relevant!)

Metrics that I?m looking for are things like:
-New registered users per month
-Total registered users per month
-Conversion rates of free users to paying users
-Acquisition cost per paying user
-Acquisition cost per free user

Please be sensitive to the usefulness of your answers.  For example, I
don?t care how many new Hotmail users there are every month, but the
conversion rate from free to paying user is interesting.  I?d also
like to understand any quantitative difference between ?joe?s web
email? and ?yahoo mail? and the different metrics (aside from the
obvious.)  ie. does yahoo mail get ?advertised? 100x more often than
joe?s and so joe has 100 times less users, or is it something else?

Most of this information I?ve gleaned from press releases or articles
on web marketing, etc. but I have not been satisfied with the data
I?ve come up with.  The number one problem is that it?s not terribly
up to date ? which in this sector makes a HUGE difference.  I?d like
to have someone else put his or her eyes on the problem.