That is what RFC6724 recommends as the default:

> Another effect of the default policy table is to prefer communication using IPv6 addresses to communication using IPv4 addresses, if matching source addresses are available.

https://tools.ietf.org/html/rfc6724#section-2.1

Many operating systems now prioritize IPv6 over IPv4. Otherwise there wouldn't be much IPv6 traffic at all.

If only v6 is available there is no point for them to include the site in their general index.

If both v4 and v6 access is available, what is the extra value of doing the indexing using v6 rather than v4? (The indexing process itself would add very little to the global usage of v6, comparatively speaking.)

In that case, what is the extra value of having ipv6 at all?

And Facebook seems to prefer ipv6, I guess there's a reason for that.

Facebook engineers also declared Mark Zuckerberg dead the other week. Maybe there's no deeper lesson here?

http://www.theregister.co.uk/2016/11/11/facebook_zuckerberg_...

Because otherwise we don't need ipv6. Google advertised the use of ipv6 heavily. If everything still works fine with ipv4 and NATs, why should we ever switch? It's a serious question, I know that the ipv4 space is somehow exhausted, but I can still get very cheap ip addresses (one is included in every $5 vps at major providers).

If even Google with its huge network of crawlers doesn't see the need to communicate via ipv6, why should there be any demand?