Facebook Inc. /zigman2/quotes/205064656/composite FB +0.52% just dove into payment systems with a service across Facebook, Messenger, Instagram, and WhatsApp. Facebook Pay, announced in a blog post Tuesday, consolidates its massive social media applications more tightly at a time when politicians and some lawmakers are calling for the company to be broken up. "Facebook Pay is part of our ongoing work to make commerce more convenient, accessible and secure for people on our apps," Deborah Liu, vice president of marketplace and commerce, said in the post. The payment-processing service, considered a competitor to Apple Inc.'s /zigman2/quotes/202934861/composite AAPL +1.49% Apple Pay and Square Inc. /zigman2/quotes/205989440/composite SQ +1.40% , begins rolling out this week on Facebook and Messenger. Facebook shares are up 48% this year. The S&P 500 index /zigman2/quotes/210599714/realtime SPX +0.95% is up 23% this year. Facebook's shares rose 2.6% on Tuesday, while Square's stock fell 3.5%.