Spammers have taken over YouTube's "Invite Your Friends" feature and are using it to send out large amounts of spam. The messages look like they are coming from "service@youtube.com" but are really come-ons for everything from online pharmacies to loan scams and everything in between. The spammers are using the feature to get around spam filters, the same principle behind those fake messages scammers send that look like they are from such legit sources as eBay, Bank of America, Citibank, and Paypal. Here is a screenshot of one of the YouTube spams. No word yet from YouTube about the problem, so your best bet is to place the service@youtube.com addy on your blocked senders list until YouTube fixes the issue.