South Korea's top mobile service provider SK Telecom Co. apologized Friday for an overnight service failure observed throughout the country, which left its 20 million users unable to use normal services. "We deeply apologize for any inconvenience caused by the service failure," the company said in its press release. "We will provide users with more details on compensation in the future."    The announcement came as SK Telecom's mobile network partially suffered a failure on Thursday night through early Friday morning, with its subscribers having problems in making calls and downloading data such as emails and maps.