Bloomberg NewsTwitter has struggled to line up a steady revenue stream.Twitter Inc. is exploring a subscription-based premium service for professionals, which could create an important new revenue stream for the beleaguered company.Twitter reported 319 million monthly active users in its quarterly earnings report last month, but user growth has been slow over the past few years, falling far behind rival social network Facebook Inc. FB, -0.04% . It has also struggled to turn a profit amid middling ad revenue.To find out the viability of a subscription service, Twitter TWTR, -0.33% is conducting a survey among some users “to assess the interest in a new, more enhanced version of Tweetdeck,” spokeswoman Brielle Villablanca said in a statement Thursday. The Tweetdeck app provides a more customizable interface, and was acquired by Twitter in 2011.“We’re exploring several ways to make Tweetdeck even more valuable for professionals,” Villablanca said.via