The HHVM discussion in the context of wordpress is mostly irrelevant to 99% of wordpress sites. The reason for the existence of HHVM is to facilitate a fast code updates to a very large amount of servers. The comparison against the performance of PHP is nice but the real competition (at least internally in FB) of HHVM is against compiling the PHP code into simple executable.
So it will benefit thing like wordpress.com, maybe even wordpress.org, but for most site it will not show any improvment whatever.

keep in mind that the biggest bottle-neck in wordpress is reading from the DB and HHVM do not help with that, so you will still need a caching plugin which will give you a 100x performance boost so why bother at all with the HHVM 3.5x boost that you will enjoy only once an hour when the cached version of the page is generated.

FB are in a relatively unique position as most of their traffic is by logged in users therefor they have very limited ability to do aggressive caching. This is probably not true for even wordpress.com.