By Marc Vartabedian Source: Wall Street Journal Shares of Twitter Inc. TWTR -8.03% and Facebook Inc. FB -2.19% endured a surprise rout this week, as investors were rattled by signs that users are souring on the social-media stalwarts. Twitter said Friday that its number of monthly users dropped in the second quarter and could continue to fall as it purges fake accounts, results that echoed Facebook’s […]