switching between ты and вы

[lovimoment.livejournal.com]

I saw a sign the other day, an ad for a cell phone company: "Подключи друга и получите $10."
[Translation: Sign up (2nd person singular) and receive (2nd person plural) ten dollars.]

I know this is ad-speak, but I'm wondering about the grammaticality of this. Does this look wrong to a native speaker?

Also, why are some ads in 2nd person singular and some in 2nd person plural? Is one or the other better?