The company said in a Securities and Exchange Commission filing that it received a subpoena from the FTC "relating to a review by the FTC of Google's business practices, including search and advertising." Google also said in a blog post that it was under investigation, but that "it's still unclear exactly what the FTC's concerns are."
Without more information, it's impossible to know where this investigation could lead. The FTC's concerns could stem from relatively narrow issues like Google's "house ads" for its own services -- which were flagged as problematic last year by Santa Clara University law professor Eric Goldman -- to practices that are more central to Google's search business.
Certainly some companies out there would like Google to answer for decisions that have had an impact on their revenue. One company, myTriggers, is currently suing Google in Ohio for allegedly violating that state's antitrust laws. myTriggers says its traffic fell after its quality score was lowered by Google, which effectively raised the price of pay-per-click ads. (That case, however, is complicated by the fact that myTriggers previously said that Google only took action after a server crash caused myTriggers' site to go down.)
The FTC investigation announced this week might be new, but Google has previously come under scrutiny by the government. In 2008, Google backed out of a plan to power some paid search ads for Yahoo after the Department of Justice threatened to sue to block the deal. Since then, however, Google doesn't seem to have made many new friends. Entertainment companies complain louder than ever that Google enables piracy, and businesses are fighting Google in court for allowing trademarks to trigger search ads. Additionally, privacy advocates criticize Google for a variety of issues including retaining logs that tie IP addresses to search queries, its botched Buzz rollout, and failure to create a do-not-track header for Chrome.
None of those more recent criticisms might be relevant to the FTC's current probe. But they might help to foster a perception that Google has grown powerful enough to warrant a closer look.