Microsoft has recently lashed out at Google with aggressive ads and an EU patent complaint, but the company says that it's not involved in any antitrust complaint, formal or informal, over Google+. Previously, sources told Reuters that the company was one of several to complain to EU antitrust regulators about the social networking tool. At the time, Microsoft said it would not comment on "rumors or speculation," but now a spokesperson has confirmed with us that "Microsoft has not complained to the European Commission about Google+."
Regardless of Microsoft's involvement, it's still possible that Google+ will come up in existing antitrust investigations. Another source told Reuters that the US Federal Trade Commission has expanded the scope of its case to examine Google+ as well as the Google search engine, and the European Commission is currently looking into claims by Microsoft and others that Google has abused its dominant web search position.