I'm not sure how they'd be doing that and still giving each tweet a number so close to all the others. I have no idea where you see a "public timeline" as people mentioned above, so I just did a search for "i"... the following tweets were about a second apart.
2088840357 2088840362 (difference of 5, and that's without a real public timeline where you could easily prove each tweet takes the next number in a sequence)
So it isn't as if they're using UUIDs or something unique... they very much seem to be relying on a single counter (currently).
For a first guess, if there were 5 writing dbs, each could write every 5th id, starting at a different index (0,1,2,3,4).
You'd end up with globally unique ids, and as long as your writes were (approx.) evenly distributed, all 5 sequences would be (approx.) close to each other.
Who knows, they might have 100 writable dbs, with only 70% up at any point in time, meaning only 70% of all integers are actually used.. anyway, I don't know, I can just imagine them doing something like this if they wanted certain things from the system.
2088840357 2088840362 (difference of 5, and that's without a real public timeline where you could easily prove each tweet takes the next number in a sequence)
So it isn't as if they're using UUIDs or something unique... they very much seem to be relying on a single counter (currently).