I met the same problem at first time, I was just sending friendship request believing that it's the only option here; I could find just 3 guys who are serious and I could talk with, but they are not available always in my activity time (because of the time zone as Nicolas said).But, before a couple of months I discovered the groups & answers sections. I started to post and answer , this way I could express myself ,doing a nice debates ,arguing with guys who have different opinions and show many aspects of my personality & interests .So , as a result many persons sent me friendship requests ,and I found many of them nice and very interesting people. I think that to find a nice group of language partners takes few days of activity and more originality in your posts, No Copy/paste.
and also , there is something you will face it -as a female- is : the frustrated & Obsessed guys ,who are just seeking in vain for a dates here, so ignoring them is the best way .
Good luck in our Community,