over the next five years, marking a sharp slowdown from high-double-digit growth, said U.K.-based market research firm Informa in a report published in May.Other European wireless firms, such as in Germany, are expected to offer the Joyn messaging service.AT&T (T), Verizon Wireless and Sprint Nextel (S) have fared better in the U.S. vs. WhatsApp and other free social networking-based services, tho