How do you think of its business model? Keep it free to get a big user base then sell it to huge enterprise? Like Whatsapp and Facebook?

I guess they have several more years to introduce something. About the business model and monetization, the Telegram FAQ [1] has long stated this:

>Q: How are you going to make money out of this? > We believe in fast and secure messaging that is also 100% free.

> Pavel Durov, who shares our vision, supplied Telegram with a generous donation through his Digital Fortress fund, so we have quite enough money for the time being. If Telegram runs out, we'll invite our users to donate and add non-essential paid options to break even. But making profits will never be a goal for Telegram.

[1]: https://telegram.org/faq#q-how-are-you-going-to-make-money-o...

Line sells stickers . that's a very effective strategy

Well, they do not aim to earn money out of Telegram, so I guess that'll just work fine if they sell some stickers or things like that. The thing about Whatsapp and others is that they are a profit minded company, Telegram isn't, they just need money to keep it running