That's not true all the time. There are girls who get tons and tons of messages. Also people sometimes are busy , doing stuff professionally or academically , on vacations , whatever , and they just forget. Heck , i forget to reply sometimes. I read the message , i think to myself i'll respond later and i forget. Nobody expects to leave such a big impression on facebook messages that the girl will remember you for eternity.
Sure , if the conversation is flowing naturally and there's a silence after asking something like that ..its kinda obvious. Still , what's the problem off messaging back ? Yay , another silence. Well now you know...