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I noticed that 'host google.com' returns a lot of IP addresses. But 'host facebook.com' returns only one. I would have figured both to either use one, or both to use several since they're both incredibly popular.

What's the reason for this?

Edit: Maybe it was a poorly worded question. I was operating under the assumption that services had numerous IP:s for performance reasons. I realize now that multiple IP:s can be because they support multiple protocols, socket types etc.

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  • You'd have to ask them. We are neither google nor facebook, and, thus cannot provide an authoritative answer.
    – Ricky
    Sep 6 '15 at 22:27
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Facebook might have only one IP, but it can 'represent' different physical servers. For example, in North America it would get you to datacenter in North America if it's up and running. In Europe it would get you to DCin Europe, i.e closest to you. This is anycasting and is governed by BGP.

Google might have multiple A records in DNS for one domain. And this is a DNS round-robin, which is another redundancy mechanism.

This might be a bird view on very complex problem of creating a redundant service. You can start with wiki https://en.wikipedia.org/wiki/Anycast and https://en.wikipedia.org/wiki/Round-robin_DNS

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