Why girls reject me all the time? & ignoring her texts

I`m dating girls through a social media(Facebook). I send friend request to unknown girls and i added them. after adding few days back i send "Hi". So girls reply some not. if not i says Hi Hi Hi ,like wise 10 20 msgs until they reply. Probably they dont reply me at all.

Once she replies me. i start conversation. probably i ask a question she answer it. once i finish asking they will never ask from me back. there are some girls who keep chatting. all of sudden i go offline and try to ignore them my expectation is they ll come behind me. but truth is they are not.

Can anyone explain why is that?
Am i doing anything wrong?
Some prof advises pls..
when to ignore her? how?

Thank you.