This is what is written on official telegram app website
Q: How are you going to make money out of this?
We believe in fast and secure messaging that is also 100% free.
Commercial companies frequently face the need to compromise their values for financial gain. This is why we made Telegram a non-commercial project. Telegram is not intended to bring revenue, it will never sell ads or accept outside investment. It also cannot be sold. We're not building a “user base”, we are building a messenger for the people.
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 or add non-essential paid options.