This is actually doable without Hubspot adding new features. You can use the Hubspot API + Facebook API to do the merge of lists.
Some tools such as AdWaffle (https://adwaffle.com) enable to do that out of the box (also can be used to create Value-Based Lookalike Audiences based on the Hubpsot Lead Score of each contact, which is pretty cool)
Hi @frankschouten - that's correct. We're working on bringing some audience creation / syncing functionality into the core Marketing Hub over the next several months, but the add-on will continue to have more advanced audiences and a higher number of simultaneously syncing audiences.