Facebook Down - Facebook Error DNS failure

It looks like the Facebook website is down again. The error being given varies from
Service Unavailable - DNS failure / DNS error, The server is temporarily unable to service your request. Please try again later to Zero size error amongst others but other websites do not appear affected. The same problem apparently occurred yesterday.
It is very unusual for a multi-country website such as Facebook to ever be down completely and unreachable for DNS. Is this a Facebook server error or some DDoS attack?

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