UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of Report (Date of earliest event reported): November 4, 2011

LoopNet, Inc.

(Exact name of registrant as specified in its charter)

LoopNet, Inc.

185 Berry Street, Suite 4000

San Francisco, CA 94107

(Address of principal executive offices, with zip code)

(415) 243-4200

(Registrant’s telephone number, including area code)

Not Applicable

(Former name or former address, if changed since last report.)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

As previously announced, on June 30, 2011, LoopNet, Inc. (the “Company”) and the CoStar Group, Inc. (“CoStar”) each received a Request for Additional Information (commonly referred to as a “second request”) from the U.S. Federal Trade Commission (“FTC”) with respect to the proposed acquisition of the Company by CoStar (the “merger”) originally announced on April 27, 2011. In addition, as previously announced on October 26, 2011, at the request of the FTC, CoStar and the Company have agreed to extend the waiting period imposed by the Hart-Scott-Rodino Act (the “HSR Act”) from 30 to 60 days after the date of substantial compliance with the second request unless that period is extended voluntarily by the parties or terminated sooner by the FTC.

On November 4, 2011, each of the Company and CoStar certified as to its substantial compliance with the second request. Completion of the merger remains subject to the expiration or termination of the waiting period under the HSR Act and other customary closing conditions. The Company remains committed to working with the FTC as it conducts its review of the merger.

Cautionary Statements Regarding Forward-Looking Statements.

This Current Report on Form 8-K and the information incorporated by reference herein contains “forward-looking statements” within the meaning of the Private Securities Litigation Reform Act of 1995. Such statements include, but are not limited to, statements about the timing of the merger, the companies’ plans, objectives, expectations and intentions and other statements including words such as “anticipate,” “may,” “believe,” “expect,” “intend,” “will,” “should,” “plan,” “estimate,” “predict,” “continue” and “potential” or the negative of these terms or other comparable terminology. Such statements are based upon the current beliefs and expectations of management of the Company and CoStar and are subject to significant risks and uncertainties. Actual results may differ materially from the results anticipated in the forward-looking statements. The following factors, among others, could cause or contribute to such differences: the possibility that the FTC will request additional extensions to the waiting period imposed by the HSR Act; the possibility that conditions, divestitures or changes relating to the operations or assets of the Company and CoStar will be required to obtain required governmental clearances or approvals; the possibility that the merger does not close, including, but not limited to, due to the failure to obtain governmental clearances or approvals; the risk that expected cost savings or other synergies from the merger may not be fully realized or may take longer to realize than expected; the risk that the businesses of the Company and CoStar may not be combined successfully or in a timely and cost-efficient manner; and the risk that business disruption relating to the merger may be greater than expected. Additional factors that could cause results to differ materially from those anticipated in the forward-looking statements can be found in the Company’s Report on Form 10-K for the year ended December 31, 2010 and Quarterly Report on Form 10-Q for the quarter ended September 30, 2011, and CoStar’s Annual Report on Form 10-K for the year ended December 31, 2010 and Quarterly Report on Form 10-Q for the quarter ended September 30, 2011, each filed with the SEC, including in the “Risk Factors” section of each of these filings, and each company’s other filings with the SEC available at the SEC’s website (www.sec.gov). Neither the Company nor CoStar undertakes any obligation to update or revise any forward-looking statements, whether as a result of new information, future events or otherwise.

Additional Information.

In connection with the proposed acquisition of the Company, CoStar filed with the SEC, and the SEC declared effective, a Registration Statement on Form S-4 that included a preliminary proxy statement of the Company that also constituted a preliminary prospectus of CoStar. On June 7, 2011, CoStar filed a final prospectus and the Company filed a definitive proxy statement and the Company began mailing the proxy statement/prospectus to the Company’s stockholders, who approved the proposed transaction on July 11, 2011. Investors and security holders are encouraged to read these materials as well as any other relevant documents filed with the SEC regarding the proposed transaction, because they contain important information regarding the proposed transaction. Copies of all documents filed with the SEC regarding the proposed transaction, including the proxy statement/prospectus, may be obtained free of charge at the SEC’s website, www.sec.gov. Copies of the proxy statement/prospectus and the filings with the SEC that are incorporated by reference in the proxy statement/prospectus can also be obtained, free of charge, by directing a request by mail to LoopNet, Inc., Attn: Secretary, 185 Berry Street, Suite 4000, San Francisco, California 94107, by telephone at (415) 243-4200, from the investor relations section of the Company’s website at http://www.loopnet.com or from CoStar’s website, www.costar.com/Investors.aspx under the tab “SEC Filings.”

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.