WhatsApp was bought because it was popular in India and Brazil, markets that have historically been weaker for Facebook Messenger. Together, they ensured that Facebook would get a nigh-insurmountable lead in marketshare over Google in the messaging space [1].

Twitter in neither a threat to anyone, nor a strong candidate for a defensive purchase [2][3][4]. The two companies are hardly comparable.

[1] https://news.ycombinator.com/item?id=13465483 [2] https://news.ycombinator.com/item?id=11914620 [3] https://news.ycombinator.com/item?id=12420732 [4] https://news.ycombinator.com/item?id=12083975