Digg, Stumble, Del.icio.us,.... do they really work ?

My apologies if this is posted in the wrong forum, but I felt it was the most appropriate one for this discussion

I've recently seen really many members requesting from others to digg, stumble or their site for them

Never really tried placing any of my sites on any of them, but do they really work in grabbing traffic ?

To be honest, I've never seen anybody using these services except webmasters promoting their sites.
Do these sites get alot of traffic from regular users ?
And are they really worth all the time and effort spent on them trying to get dugg or stumbled or whatever

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