Robert Cyran is a Reuters Breakingviews columnist

Facebook's defensive purchase of Instagram raises a red flag. Online photos are supposed to be a core Facebook competence. Paying $US1 billion for the popular picture-sharing app may boost the social network in mobile.

But paying over the odds for revenue-free rivals is usually the hallmark of anxious, mature firms - not a growth company seeking to go public at a $US100 billion valuation.

It's impossible to say exactly what Facebook gets for the oodles of cash and stock it is handing over to Instagram, founded just two years ago by Kevin Systrom and Mike Krieger. Traditional metrics don't apply - Instagram is just embarking on an actual business plan, and the firm was worth just $US20 million a year ago. What it does have are lots of users – more than 30 million – and super-fast growth. More than 1 million more users signed up in 12 hours for its new Android app last week.

Facebook is clearly acquiring the firm for other reasons. People are spending an increasing amount of time connecting via their mobile phones. This shift is worrying for the formerly desktop-focused Facebook, whose own prospectus warns of the risks to its business of an increasingly mobile Internet.