Facebook is, by a
huge margin, the buggiest release software that I use every day. When I look hard, I notice at least a dozen UI bugs a week. Yet, Facebook is hugely successful -- so successful, in fact, that it's hard for me to think of ways in which it could be more successful. Your comment implies that there is some threshold that they will pass, at which point they will be forced to adopt more stable sensibilities. There are 1 billion active users [1], so I ask: if they haven't gotten to this point, when exactly will they?
I don't believe the lack of quality in Facebook code poses an existential threat to Facebook, and I don't believe it ever will:
(1) Facebook code is not Google code, but it is not bad code, it's just not needlessly good code. If Google is consistently buggy, anyone can switch to Bing. On the other hand, switching from Facebook is really hard. Their core services are not easily replaceable. People are willing to put up with more. Note that this is even true in the case of businesses: Facebook is horrible to develop against, but developers don't have a choice.
As the network grows, this cost will be higher, which means Facebook can throw its weight around more, not less. This is exactly opposite of the situation you predict. The only thing that can really reverse this is if there is a viable competitor, but the increased growth of Facebook makes this increasingly unlikely. It is clear, in any event, that G+ is not quite there.
(2) By maintaining a lower bar, Facebook actually has an advantage. It is a website, and can fix its product at will. Code can be deployed quickly and rolled back accordingly, making failure cheap. This means they can concentrate on things that really matter like user acquisition.
(3) Facebook does not run nuclear reactors or bank systems. There is a high incentive to switch from buggy bank systems. There is little incentive to switch from buggy social network websites.
footnotes:
[1] whatever that means