I've met probably over 100 women from various dating sites and had message exchanges with around 5 times that many, and I've never really figured out the motive behind that question. My favourite response is " ... well its early days yet I've only been on a few says/weeks (as appropriate)..." followed by a swift change of subject. This gives them nothing they can interpret negatively (which most more elaborate answers will) and underlines the fact that your not a serial on line dater (even if you are) most sites thankfully don't show how long youve been signed up.

As for more general tactics: try to go for a direct phone/email/skype/msn contacts exchange after 3 or so medium sized messages (say 100 -150 words). Try to pace your responses to one every 24/48 hours. On your third response just end it with "anyway I think we should talk directly, you can get me on #999999 / <email/msn/skype/--whatever-->" always give your number as a minimum and whatever else takes your fancy. Don't directly ask for theirs or hint in any way that you expect them reciprocate. Personally I avoid facebook etc as it just gives too much away. Really the volunteering your number first tactic is just pure gold and in my experience gets at least an 80% followup rate of them reciprocating or contacting you via one of said means.