Marketing strategy in Social network and their offers

I was going through the various networks and their advertising plans, just got a thinking how much they are effective.

Stumble upon : $0.05 - Per Each Visitor - Seems little costly to me

Mybln : 50k impression for $5 and 5k free impressions

imfaceplate : 5,000 credits     $14.97 base offer, you use it for many purpose.

Apsense : They use their own credit system...so the more you contribute the more you can promote, they have buying options as well.

Looking imfaceplate and Mybln more powerful to me as they are growing rapidly and they have effective user base.