If you haven't yet had the great privilege of receiving the rather unconvincing private message that is doing the rounds, then your turn will no doubt come. It sounds friendly in a teenagery sort of way and invites you to click on a link, which hopefully we are all internet-wise enough to ignore.
If you get this or any other spam, you have the option - and some might suggest also the duty towards fellow multiplyers - to report this so that the user can be deleted. I have recently discovered that there is a way of doing this without visiting the perpetrator's site - use this link, just substitute "useridgoeshere" with the spammer's user ID:
http://multiply.com/info/inquiry?report_user=USERIDGOESHERE
And by the way, you may or may not be aware that in your profile settings you can choose who can see your email address and other basic contact details - if you set it to Contacts, which is the minimum level allowed, then the spam you get will only be in the form of Personal Messages delivered via Multiply and not in the form of a direct email from the spammer.
P.S. Reporting abuse is of course not only for spammers but for anyone else who breaches the terms of use. You can also block a specific user if they cause you aggro, though so far I must say I haven't come across any nastiness here.