Facebook and Instagram are more of a walled garden, like Quora, but there is a lot of junk there anyway.
It's sad for the WWW, but I don't really think it is a fundamental problem for search engines. In fact Twitter for example gives a direct pipe to Google. If you tweet something, it is immediately findable. Similar for StackExchange, but there I think the site is so "small" that Google can afford to just continuously index it.