I disagree. The SLA ought to be made on a per-customer basis. 1% of users affected would mean 1% of users would be entitled to refunds/remedies that the SLA prescribes.
Their SLAs are short and readable. It is worth reading a few of them, especially if you have a SaaS and are thinking about your own SLAs. Just search Google for [google $product sla] to find them.