Attractive girls can and do send out messages. They still search the site and do all the normal shit even if they have 60 unread emails. That is why having a profile that stands out is the first and most important step to online dating. From talking to various women I went on dates with from these sites basically once they get a message the skim it first, go to the dude's profile and see if they want to respond. Some said they first checked to see if they thought the guy was attractive before reading a single word. Since online dating is all about the law of probability and being quick you basically need to send a first message that is so short she won't skim it (skimming generally means you're bored and don't want to read it all) and have a profile that is also easily readable. Personally I found a short intro sentence about me, followed by a list with 6-10 bullet points about me and a short outro worked best.