FIGHT! Instagram, the mobile photo-sharing service which was acquired by Facebook earlier this year, has been trading blows with Twitter. The scuffle between the two social networks has intensified as both Twitter and Instagram have adjusted their policies and released new versions of their smartphone apps. What are they fighting about, and why?

Instagram used to play nicely with Twitter. But hostilities broke out in July after Instagram was bought by Facebook. This prompted Twitter to disable a feature which let Instagram users import their lists of friends from Twitter; this was, in effect, importing Twitter friendships into the Facebook universe. Now Instagram has shot back. From December 10th Instagram images are no longer visible in tweets. Twitter users must instead visit Instagram’s own website in order to see and comment on them. The same day, Twitter responded by releasing a new version of its app, which allows styling of images using a range of filters — a feature popularised by Instagram. On December 11th Instagram retaliated by beefing up the camera on its own app.

At the root of the conflict between the two social networks is the desire to maintain control over their respective user bases: around 100m active users for Instagram and 150m for Twitter. To begin with both networks simply wanted to sign up as many users as possible, so they were more inclined to be open and co-operative. Having attracted tens of millions of users, they now wish to maximise the amount of time spent on their respective services, and hence the opportunity to display advertising. Twitter is thought to be preparing for an initial public offering next year, and Facebook wants to extract as much value as possible from Instagram, for which it paid over $700m. So each network is trying to prevent the other from appropriating the valuable data it has amassed from users. Hence Twitter’s decision to deny Instagram access to its database of friendships, and Instagram’s decision to deny Twitter access to its photos. (Speaking at a conference last week Kevin Systrom, Instagram's boss, called this "an evolution of where we want links to our content to go" — a striking turn of phrase given that the content is provided entirely by users.)

The competition between the two services is good in some ways, because it is forcing them to add new features and improve their offerings. But this benefit is outweighed by the drawback that Instagram and Twitter are becoming steadily more closed and restrictive. The fight between them is merely the latest illustration of the fact that when it comes to online social networks, the interests of the companies that run them do not always align (and may well conflict) with the interests of users. For services that wish to make money from advertising, it makes sense to be open initially — but then, having attracted a large base of users, to start building walls around them.