The U.S. Federal Trade Commission is investigating Facebook's (News - Alert) recent purchase of the mobile photo-sharing service, Instagram and is reaching out to its competitors, Google and Twitter, for more information on the deal, Reuters (News - Alert) reports.

The investigation does not necessarily mean that there was anything improper about Facebook’s $1 billion acquisition of the company, announced in April. The FTC (News - Alert) routinely investigates large acquisitions like this one.

It is still unclear what information the investigators are actually looking for.

Facebook is on its way to completing the acquisitions of the company by the end of the second quarter of 2011. In addition, the social networking giant is preparing for its IPO, which could happen as soon as next week. Facebook is expected to raise at least $12 billion.

The impetus for the acquisition of Instagram is the shift from desktops to mobile phones. Instagram, with its focus on smartphones, initially the iPhone (News - Alert) and then Android, made it seem like a good target for an acquisition.

Google and Twitter, for their part, have stayed mum about the investigation. No representative from either company has spoken to the press about anything they might have been asked so far.

Facebook has struggled to generate ad revenue from mobile use. “They have to figure how to monetize their mobile audience in the short term,” Jed Williams, an analyst with analyst firm BIA/Kelsey, said earlier in May. “They haven’t done that to date, and they have to do it in a way that doesn’t disrupt or shatter the user experience.”

Facebook, in addition to its acquisition of Instagram and its upcoming IPO, has also launched its own app store, similar to both Apple's (News - Alert) and Google's. The availability of paid apps may give Facebook a way to make more money off of the increasing number of mobile users as they move to their phones away from their computers.