So were you thinking that you were actually giving away space from your own account? In such a case, how would it work if 100 people signed up using that code?
As moua has said, this is what really happens, though it's not entirely clear whether the bonus just applies to the signup amount or whether it would also apply to your "growth" amounts.
Really, it's hard to see how any of these disk or bandwidth codes makes financial sense - especially with the current 500GB initial allocation. You should also remember that we're getting 2GB extra space every week, which comes out to around 100GB a year, so if you think you might need extra disk space in the future, you'll probably have it by the time you need it.
Then again, everyone's needs are different and I suppose there are sites out there that are going to host large downloads and will need the extra hosting space - though I suspect that you'd run out of bandwidth before you ran out of disk space.
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