When the CEOs of big companies buy smaller startups in a grab for their members, the executives know there’s a very important, very formulaic promise they must make: Nothing will change. Mark Zuckerberg trotted out this line when he acquired Instagram. (“Were committed to building and growing Instagram independently,” he assured.) And he parroted it again last week after paying $19 billion for WhatsApp. The messaging service will, Zuckerberg wrote, “continue to operate independently within Facebook.” The arrangement sounds nice, but the reality isnt so rosy. …