I'll stop using Opera (currently my default browser) if facebook acquires it. Such is the level of trust I have over facebook.
Call me a zealot, but if there's one piece of software that I feel ought to be open source, it's a critical centerpiece of modern communication like the web browser. As such, Opera is already not an option for me. In fact, a Facebook acquisition could even change that if the new regime decided to produce a Chromium equivalent.
I've been using Opera since its paid days, and despite having switched browsers every year or so, I've still gravitated back to Opera as my day-to-day browsing browser. That's the value of their brand.