It's not indexed in realtime. (tweets from 20min ago are still not searchable) It has no interactivity either which makes the problem few orders of magnitude easier. And they don't care about subscriptions.

Implementing search: just chuck the last hour of fire hose into s3 and update the ngrams.

Implementing real timeish view: ok, so we need to distribute+replicate the data so timeline queries can happen in parallel and get fresh info depending on subscriptions.

Implementing the number of likes ticking up as you watch the tweet: basically alien technology. (I assume it's optimised somehow because otherwise that would convert a single tweet view into 1 + however long someone stays on the page / 2s times the load in naive implementation)