Thankfully nothing is 32-bit so no worries about integer overflows. That would cause huge headaches everywhere on the PHP side. In MySQL, a regular unsigned INT column does have that limitation (roughly max 4.5 billion for unsigned, 2.2 signed), so BIGINT must be used there (Twitter had to do the same). Where it gets interesting is PHP doesn't support unsigned integers, so with 64 bit your max integer in PHP is 9,223,372,036,854,775,807, whereas in MySQL an unsigned 64 bit int is double that. I think it's safe to say though that neither Tumblr nor WordPress, even combined, would ever have more posts than atoms on Earth =)
If my math is correct, you're a wee bit off.
9,223,372,036,854,775,807 is 2^63, which is roughly 10^19. The Avogadro constant, which is about 6 * 10^23, is the number of particles (atoms / molecules) in a mole of substance, which for atoms amounts to (atom number of element) grams of mass (so e.g. 12g of C12 is a mole).
http://highscalability.com/blog/2012/2/13/tumblr-architectur...
Currently I think Tumblr stores all posts across all sites in one big table? WP.com does different tables per site.
I also think Tumblr's post ids are often above php's int max for 32-bit systems ( 2,147,483,647 ) -- I know I've seen some issues trying to parse tumblr's post ids to integers rather than strings on some old servers years ago.
For an overview of how our systems are run here's Barry, our head sysadmin, talking about six years ago on how the wpcom infastructure is structured:
https://www.youtube.com/watch?v=57EJ8KDDBH0
It's changed somewhat, but not much conceptually. It's a really fascinating talk and I'd encourage anyone curious on massive scale data to give it a look, and see precisely what can be managed with determination and mysql and coffee.