Answer
A signed 32-bit integer tops out at about 2.1 billion (2³¹), and an unsigned one at about 4.3 billion (2³²). A busy table of events, messages or likes can pass that in months, and changing the type of a primary key on a huge table is a painful migration. 2⁶³ is about 9.2 × 10¹⁸, which never runs out.