SoftBank Group Corp. tumbled below the value of its stake in Alibaba Group Holding Ltd. amid growing concerns about the Japanese company's other assets, including struggling U.S. wireless carrier Sprint Corp. SoftBank's market capitalization stood at 5.89 trillion yen Friday after its stock rebounded 8 percent after a four-day losing streak triggered by rising pessimism about Sprint's ability to pay down debt. That still lags the $56 billion that its stake in Alibaba is worth, according to its own website.