Directly addressable just means it has a globally unique and routable IP address. Ordinary corporate IPv4 networks use private networks according to RFC 1918. Those addresses can be used by anybody, but they really should be unique inside a single organization. At Facebook's scale, trying to keep all those RFC 1918 addresses unique and having enough addresses for every use was becoming too difficult.
In IPv6, the idea is each device actually has multiple addresses. It has a link-local unicast address, a link-local multicast address, and ideally one or more globally routable addresses. The link-local addresses are to replace semi-IP protocols like ARP, but they also can be used on their own. The globally routable addresses could include static addresses, but for privacy they usually use one or more temporary, but still globally routable, addresses for use for about 24 hours. They automatically allocate and discard these temporary addresses.
Every IPv6 implementation I know uses globally unique and routable addresses. Current versions of the major phone and desktop operating systems support IPv6 natively. When the major ISPs upgrade to IPv6, all the ones I've seen give globally unique and routable addresses.
However, having a directly addressable IPv6 implementation doesn't mean everyone can access your computer. Firewalls can still block connections that you haven't initiated. It's still better to use IPv6, because the vast address space makes network management much, much easier.