Note: I don't use Facebook's app often, and have no context for what the actual issues are.
Mobile decision making is made mostly by consumers, not business. These devices are for people to play angry birds and check email. Android and iOS aren't under a lot of pressure to provide a stable and well audited environment. Sales are good, apps are selling, etc why rock the boat?
I think as these platforms mature things might change, but from what I can tell, there's still a major disincentive to move off the Windows ecosystem, especially when I can have a quasi-mobile experience with the Surface line. Its too much amateur hour in mobile. The post-PC cheerleading, it seems, was way premature.
Rule of thumb is an app can allocate approximate 50% of total device memory before crashing. But there's no hard limit and it depends on your allocation pattern. If you request too much too fast it'll crash as well. Sometimes you have to let it fail and spin for 3 seconds trying to allocate again hoping it'll go through. Sometimes it will and sometimes it won't!