The whole "pursuit" issue is really rather annoying to us guys. Mainly because we've all done it, and 9 times out of 10 the girl is just stringing us along. I have no problem with the concept of pursuing, as long as it is mutual. There HAS to be some pursuit back so that we know we're not wasting our time. If I send a message to someone on this site it's because I'm interested, not just to say hi. I would like to think if there is a response other than "not interested" then that means she's interested as well.
Also, don't read in to how long it takes for someone to ask for your phone number. In fact, don't read in to anything, we're not as complex as you would like to think. Maybe that person just moves slow. Maybe if you want to take it to the next level you should let him know by saying "Hey, do you think we should exchange numbers" or "What's your last name? I'd like to add you on Facebook". Sometimes we need a little help to know that you're really interested before we put ourselves out there.
I'm not saying it's only up to the guy to pursue, I'm saying that if it wasn't important enough to either person to notice/care enough to message after a week, then there isn't anything there. I like the whole pursue thing, as I have never been pursued (I always every time end up paying for dates, buying his groceries, making the trips, or asking for his #, or otherwise do all the work) however, that's not what I was saying up there.
I would ask you not to read into anything, either, as you said; if her response is anything other than "not interested, you take that to mean she is? If the reply is anything other than "yes, I'm interested" I take that to mean he isn't. There is nothing to say a reply means someone is interested in you.
And I know all about slow. My last relationship I dated the guy 8 months before he would even kiss me. All I'm saying there is that when you really want to get to know someone, when you are so struck by them, you will not let them go, and if that is not apparant by whatever time you notice it's been so long between messages, then it probably wasn't meant to be. On average, I'd say this is past 10 days for me. I usually reply to a message 3-7 days after I get it, on average, but 10 days tells me: neither of us is pursuing this. And if I get another message after that, I find it a pleasant surprise rather than an anticipated reply.