While a regular Bitcoin transaction can take up to one hour to properly confirm Sendbitcoins.net allows to send and receive coins instantanously.
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A deposit is credited to your account after 4 confirmations.
4 confirmations can take up to one hour. How is this instantaneous?
Sorry, but this is just an misunderstandment. The service is kind of prepaid: You send coins to an address which are then credited to your account. (Like when you send money to Paypal)
From there on you can spend it instantanously to other users or via the Paypal IPN api, if a shop changed the address to sendbitcoins.
Cheers.