https://gizmodo.com/5632095/justin-bieber-has-dedicated-serv...
Twitter never had dedicated machines for individual users. Its just not how the infra ever worked (at least until I left). Requests landed on random boxes behind load balancers, those boxes talked to pools of memcache or mysql, etc. At no point was there ever "racks" or "machines" dedicated to specific individuals like the article claims.
That being said, some users created crazy hot shards when specific tweets went absolutely madhouse, especially Beiber and those like him.
Random twitter internals tidbit: We had a unit of measurement called a "MJ". Its the number of tweets per second that we had when the rumors of Michael Jacksons death were circling. It basically overloaded the system and had us running around on fire. It was 465 tweets a second. Within a year we had crossed a line where we never were below that number again. Hence "we are at about 12 MJ's" was jokingly used to compare "hair on fire" to every day a couple of years later. =)