I'm more concerned with end user page load speed than crawling. I find Facebook and Twitter pages annoyingly slow to load, personally. Sure, server side rendering is difficult but it's exactly the sort of hard problem that a framework ought to be able to solve for you.
Edit: actually I just went to twitter and it loaded pretty fast, so I looked at the network requests and they do server side rendering for your timeline.
Hm, looks like you are right. At least the new timeline is server-side rendered. I guess they are updating parts of UI to be rendered on the server to be faster.
Interestingly some websites like GitHub are doing the opposite: they are slowly bringing incremental ajax-based transitions to every page, some "real-time" features are implemented with a socket signal refreshing the whole page.
One of the community solutions is "FastRender" package, which is not really about rendering. What it does, it puts most of the initial data into HTML page so you don't need to wait for the websocket connection and the data arriving to render the initial page. Improves the initial page load quite significantly: https://meteorhacks.com/fast-render/
Edit: actually I just went to twitter and it loaded pretty fast, so I looked at the network requests and they do server side rendering for your timeline.