My impression for language exchange apps. I have studied English since I was 12 years old, and i have used some language apps more than 2 years.
I want to share my impression for them.

〈Hellotalk〉
I like it the best.
There are many generation's users.
It has many function, and I guess there are the most users.
It has moments function, so if you have many questions, you can get answers immediately.
But I'm not sure if the answers are correct.

〈Tandem〉
It seems like there are many mature users.
I hardly continue conversation. Thete are many users who only say"hi" or "hey beautiful!"
But it is the easiest to call.

〈Italki〉
I posted on notebook and discussion, but I always receive a couple of answers. I hardly received answers from native English speakers. 
But luckily, some teacher might correct or comment on your discussion or notebook.
Users are many generation.
I guess the lessons are the most awesome than I have ever taken.