Think of it this way: if someone taps you on the shoulder in real life, you have to respond in some way. Moreover, it is certain that they are addressing you personally - and not anyone else.
Online, by contrast, you are inviting a response from anybody might happen to be interested in your question or request. First, not everybody on the group in question will read your post. Second, not everybody will be interested in what you have to say or in the question that you raise. Third, of all those who might be interested in principle, not all will have the time to respond.
Fourth, your whole "girls especially" approach is likely to turn people off. It suggests that you are trying to hijack this site to meet girls instead of using it to improve your language skills.
There's no reason why you shouldn't want to meet girls, but you are wasting your time on sites like this. Go to the beach; show off your muscles; join a club; go into a singles' bar.
Who knows - in the course of all of that you might meet your ideal woman andlearn a language. Good luck and more power to your elbow.
If a random person just says hi to me on the street I will most probably not answer. I will also do my best to avoid them if I understand they are looking for a kind of exchange that I'm not interested in, for example to advertise or sell something to me that I don't need. I will only respond to a reasonable and well articulated query. The same rules apply to the Internet.
If what you are talking about is a barter trade system, does that mean you only send messages to people who have indicated on their profile that they want to learn your native language? Or are you complaining that people who are not in the market to learn your native language do not respond to your messages?
From my perspective, the only barter event I am attending is one that exchanges English for Spanish. I think it is easy to tell on most people´s profile what market they are attending. At first I tried to politely decline invitations to help people individually with English, but after too many people would not accept no for an answer but instead would try to make me feel guilty for declining, now I often prefer not to respond.