How to say it in English? I need help.
For example, I want to say:
It is more CONVENIENT for me to talk IN WattsApp.
I think it is more CONVENIENT to meet you in the evening.
There are many words in English that I would use - convenient, suitable, usable, useful, maybe even prefer. But word PREFER (I prefer communicating in skype) expresses that I like one thing (way) more than another one, but I want to express that I considered/I'm convinced that one thing more useful, practical, faster. What the word/construction should I use? What is the right preposition - to talk IN wattsapp, IN skype, posting ON Instagram?

Question

How to say it in English? I need help.
For example, I want to say:
It is more CONVENIENT for me to talk IN WattsApp.
I think it is more CONVENIENT to meet you in the evening. 
There are many words in English that I would use - convenient, suitable, usable, useful, maybe even prefer. But word PREFER (I prefer communicating in skype) expresses that I like one thing (way) more than another one, but I want to express that I considered/I'm convinced that one thing more useful, practical, faster. What the word/construction should I use? What is the right preposition - to talk IN wattsapp, IN skype, posting ON Instagram?

Answer

Possibly the word that would express what you want to say is "effective". 
"It is more EFFECTIVE to talk using WattsApp."
This makes it clear that it is not just that you like it better, but that you think it has real advantages.

Answer

Ну Вы правильно использовали слово convenient в смысле "мне удобно", 
а предлог зависит от глагола, talk in, post on

Answer

To start at the back:
Post ON a website. Talk on/using/in/over/with all work for communication apps. I prefer 'using' or 'on'.
WhatsApp - check your spelling.
I think that 'I prefer' is more polite. If you want to be more assertive, you could say 'I think xxx is better', or even more assertively: 'xxx is better'. Other people may have different criteria for judging, and may have different network environments, which means that different apps perform differently.
I think that 'convenient' is a good word - and for that sentence is better than any of the others.

لا تفوّت فرصة تعلّم لغة جديدة وأنت مرتاح في منزلك. تصفّح مجموعتنا المختارة من مدرّسي اللغات ذوي الخبرة وسجّل في درسك الأول الآن!