The slump continues. After plummeting earlier in the month, Facebook shares dropped to a new low Friday morning. Shares fell more than 4 percent to an all-time low of $18.23—about a 52 percent drop from its initial public offering on May 18. The reason for the tumble? Concerns are mounting about a slowdown in spending on advertising and the risks ahead as several lock-up expirations in the next year threaten to weigh on the stock.