Some people are just too shy to decline anybody's invitation to be friends. And you never know, what person you will manage to built up strong relationships with. In many cases it's just a basic politeness and unwillingness to hurt anybody's feelings.

Often friends on this site are like ships in the night, they pass by then come back into your life again. it doesn't matter how often you email or chat.

I am an italki user that has a group of friends but I had been in contact with them few times (with someone never....). Maybe it's because I don't know the right way to use this site, I mean I received their invitation, I accepted, some of them contacted me with mail, other with skype, and after this they stopped, no matter if and when they will contact me, they will be welcome.