Having turned down a $3bn sale to Facebook in 2013 - Snapchat is preparing for one of 2017's most hotly-anticipated IPOs.

The app is now valued at $25bn and its competitors have been shameless in their attempts to ape its functions.

However, given the secretive mature of the business (and its sky-high valuation) there are a lot of unknowns as the company prepares to go public.

Apart from this headline number, we know little about the company's finances. It is reported to have told investors that it took-in between $250m and $350m in advertising revenue last year - while eMarketer expects that figure to rise to more than $900m in 2017.

The Wall St Journal believes that the founders will structure the IPO in a way which allows them to keep firm control of the business.

Then there's the issue of Facebook and Instagram borrowing Snapchat gimmicks. In June 2016 it was reported that the app had 150 million active daily users. That figure has already been matched by Instagram's stories feature.

It remains to be seen if the app will continue to attract new users at pace when biggest social networks are copying some of its main selling points. Investors may fear a Twitter-style plateauing of the business down the line.