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PaidContent blogger Rafat Ali has more details on NBC's purchase of (broken by Valleywag last week). Ali says the social site sold for under $50 million — not quite the $500 million-plus that News Corp paid for third-generation social site MySpace last year.

The disappointing sale price is Tribe's punishment for staying private too long (three years, actually). After dumping a third-round investment of $3 million into the company, investors must have given up grander hopes and looked for the first decent offer.

More motives revealed after the jump.

While the venture capitalists ground their teeth, Tribe's user growth slumped, and investors Knight Ridder and the Washington Post Co. failed to launch Tribe-based local classified systems. As recently as May, Tribe CEO Jan Gullett excused her company's profit struggle, saying "We don't want to pollute with an external economic motivation."

Ali says Tribe is only useful for its community technology, which NBC property iVillage needs. TechCrunch blogger Michael Arrington congratulates Tribe just for selling before the technology's cheap as water.

NBC's motives for buying Tribe on the cheap were explained by a Valleywag guest writer: the network wants to nail the progressive demographic it's courting with iVillage and a partnership with YouTube. being bought by NBC [paidContent]
Earlier: Guest post: Why NBC is buying [Valleywag]
Earlier: Exclusive: NBC is buying Tribe — but why? [Valleywag]