Derick Rethans has a post on his site with an update about the MongoDB driver for HHVM - mostly that, because of feedback he received, it will no longer be maintained.

At the start of 2015 we began work on an HHVM driver for MongoDB, as part of our project to renew our PHP driver. Back then, HHVM was in its ascendancy and outperforming PHP 5.6 two to one. With such a huge performance difference it was reasonable to assume that many users would be switching over.

[...] With PHP 7 released, we saw very little use of the HHVM driver for MongoDB. Some months ago I did a twitter poll, where very few people were indicating that they were using HHVM—and even if they were, they would likely not choose to switch to HHVM given the current climate.

He includes some of the responses in his post ranging from people saying that HHVM only "masked slow code" for them. Another mentioned that, while Hack has some nice features, PHP moves on and integrates some of these ideas anyway. As a result of this disinclination towards HHVM on new development and the low adoption of the HHVM MongoDB extension, it will no longer be maintained. If you're a user and are interested in keeping development going, contact Derick for more information.