If part of your strategy is to grow your business virally based on word-of-mouth, I would definitely recommend Magento, which has more "Social commerce"-related plugins (and a more vibrant developer community), like Facebook and Twitter integration on login, reviews and sharing, etc.
You can check out Magento's extensions directory and marketplace over at Magento Connect: http://www.magentocommerc
Which shopping cart is better? Magento or Big Commerce?
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