Facebook Messenger (FB +7.3%) will soon no longer be integrated within Facebook's core mobile apps. Users looking to chat with friends via Messenger will have to download the standalone app (fairly popular with mobile users, but not as popular as the core app).
Mark Zuckerberg mentioned Facebook was considering such a move last November, arguing "there's more friction to replying to messages" from the core app.
Also possibly a factor: Facebook's interest in growing its mindshare in an iOS/Android-dominated mobile landscape where each app is simply one icon on a home screen. The Verge has reported Facebook plans to launch a slew of new standalone apps in 2014.
In addition, it probably hasn't escaped Facebook's attention soon-to-be-acquired WhatsApp will soon have 500M+ users for its standalone messaging apps. Nonetheless, a backlash from some Facebook users accustomed to chatting via the core app is likely.
Flurry recently estimated Facebook's various services (not counting WhatsApp) has a 17% share of time spent by U.S. consumers on mobile devices.
Earlier: Facebook gains on bullish notes; Sandberg refutes political speculation