Facebook is facing a probe from the US Federal Trade Commission following its deal to acquire photo sharing app Instagram, which could delay the closing beyond the second quarter target set by the social networking giant.

According to the Financial Times, the regulator has already begun collecting information from companies such as Google and Twitter. The investigation was said to be routine for any transaction worth more than US$68.2 million, but could still mean that it will take some time to come to fruition.

The news comes as Facebook has acknowledged its mobile app strategy still needs some work. While an increasing numbers of users are accessing its service using apps, these currently do not feature any advertising – diverting users from its sponsored, online services.