For what it's worth, I started about 7 years ago as a volunteer
moderator, then got offered a job when mods and community managers were
still making below poverty level wages (CDN$15,000 a year). Eventually I
moved on, got asked to consult for a startup, then joined a community
for a 1 man game development company and said "hey, if you need help,
let me know". I've been working with him for over 2 and a half years now
and in the last year it has led to interview offers for several jobs
with packages worth 6 figures. For someone trained in International
Development and looking at making 20-30K in my field of study, that's
pretty good from where I'm sitting. So my advice is to find a company,
join the community, act "modly" and promote the site, then approach the
company when you think the time is right. Smaller companies are easier
to get positions at, even if they are part time volunteer for a year or
two it will get you the experience you need to go to a community
management company (alchemic dream, mzinga etc) and work your way up there.

Comm and journalism will probably get you in the door faster :) I went
moderator - community manager. Lots of people go PR/Marketing -
community manager.

As for stats, I can't give you specific numbers but
http://www.onlinecommunityresearch.com/ might be able to help.

For anecdotal evidence, I work for a website that started out at 296
users when I joined (I was #297). In 2.5 years we have grown to 300,000
+ with 23,000 daily unique active users. We have a team of volunteer
graphics artists who put out a new *professional quality* game board on
average every 2 weeks (we're a casual online gaming site) a volunteer
tournament team who are currently running approximately 100 tournaments
and I manage a team of 20 or so volunteer moderators and coordinators
remotely in 4 countries.
My position and the volunteer team that it has enabled saves the company
in the ball park of $1-$2 million a year if we were to pay for all our
graphics design, tournament coordination, and moderation salaries - oh,
and I only work 50% time. So if you can convince them to invest $40-80k
into you part time to full time, and you save them $1-2 million a year,
I'd say it's not a bad deal :) Those are not hard number BTW (thus the
massive range) please don't quote me on it :)

I hope that helps both of you.