The after-hours plunge in LinkedIn Corp.'s stock LNKD, -1.95% is attracting a lot of attention, as 2.4 million shares changed hands in the 45 minutes since the closing bell, more than double the full-day average of 1.1 million shares, according to FactSet. The stock tumbled 24% to $190.84, the lowest price seen during regular trading hours since Oct. 16. Earlier, the professional networking company provided a profit and sales outlook that were well below expectations.