What do you expect from an online friendship?
You might care much about them, their lives, their health and think about them as your friends, I mean like real friends. But at last u guys are still online friends. They can't come to you, can't share with you in real life... What would happen if someday you realize that "oh, wow they just think about me as a language partner to talk with in their spare time, and they even don't care if the time is suitable for me or not...I am not someone they need care about..."
It is your choice to talk with them. And if you feel happy now, why do you think too much about "friendship"? Finally, what is friendship? What do you expect from an online friendship???