I am not sure IDs are exactly incremental in Twitter at this point. I think they are issued by blocks or something of that nature. Someone here could answer that far better.
Twitter calls them "snowflakes". It's a 64-bit ID of that consists of a tuple of sorts of a timestamp, machine code, sequence ID. They start with timestamps to mostly give them monoticity in the direction of time (ie, sort order them), but they definitely aren't simple sequence IDs.