Check
www.socialdegree.com - a blog on the innerworkings of the social networking industry.
To answer your question, you need to further define the parameteres: What does the network consists of - are they simply profiles that people fill out with pre-defined criteria with the option to ad a few pictures or are they fully customizable web spaces (like MySpace)? That will severly affect your bandwidth requirements.
What's the average length of time people spend on the your network? Again - affects bandwidth.
And you say 2 million users - what kind of time frame are you talking about? 2million registered users, but only 100K monthly site visits, or 100K registered users with 20MM monthly site visits.
To put things in perspective, on HOSTING FEES alone, YouTube eats up about $1MM a month. At the same time, on HOSTING FEES alone, MySpace eats up about $600K a month. (
www.socialdegree.com and
www.techcrunch.com).
You haven't even got into designs, web development and legal fees (which in the social networking environment, aren't that big of a deal). What you haven't considered is marketing - which despite the focus on "viral marketing" is a much larger expenditure for social networking firms than many think.