Assessing Twitter's financial health has always been a good-news, bad-news affair.

Every encouraging development also brings a qualifying “but” that takes some of the wind out of any positive momentum.

Today's earnings report was no different. Twitter's revenue of $590 million exceeded Wall Street's estimate. But: Revenue declined 4%, falling for the third straight quarter. Ad revenue fell even further, by 8%, with ad revenue in the U.S. falling 18%....