From Boing Boing:

In the popup that appeared, the NoScript extension was blocking some JavaScript. Clicking on the button to enable the code revealed that the JavaScript was hosted at — gasp — Plaxo (no linkee for them), the company that enlists its users to send spamalicious address book related emails. So from the looks of it, when you share a YouTube video with your friends, you may be unwittingly adding your friends’ email addresses to Plaxo’s databases. Slick. Or, should I say, slimy. At least it appears so. There is no confirmation that this is what is happening, but it certainly does look suspicious.

I have always been suspicious of the companies that wish to ‘backup’ your contact details. The very fact that they ask you to spam your friends to invite them to join up worries me.

Airset may be a better, safer option.

UPDATE: BoingBoing has a rebuttal from Plaxo.