Something guys should understand about not getting replies is that it isn't necessarily out of a lack of courtesy.  There are some very valid reasons women may not write back. 

For one, women get a lot of messages. That number you're thinking of? Double it. Possibly triple. If she shows cleavage, make that at least quadruple. A lot. It makes even being on certain sites practically a full-time job if they are to respond to every single one. And, as you can imagine, most women are not there looking for a job... well, not that kind of job. So instead she picks and chooses which seem the most interesting and compatible. Yes, there is a possibility she may be mistaken about whether you'd be compatible, but that is her choice to make.

Or, let's say she does reply to every single one. That means she has to be less personal, give less attention, say much less, so her chances of making a connection are also less. To those she doesn't find interesting, she says "Thanks, but no thanks." A few of the guys will accept it and move along, but many of them will go one of two ways:  Either they will take it as a challenge and start messaging incessantly with either begging or bargaining tactics. Or they will become offended and abusive. Where their first message was, at least in their minds, complimentary, now she is garbage they wouldn't fuck with someone else's dick. It doesn't take long for a girl to learn that not replying is preferable to having to put up with the bullshit that comes after turning a guy down.