A controversial open letter directed at Facebook CEO Mark Zuckerberg left another Marc in quite the pickle yesterday.

In a note entitled “Dear Mark Zuckerberg,” Dalton Caldwell, the serial founder now behind social utopia App.net, publicly chastised Facebook for negotiating in bad form and accused the company of threatening to destroy his business after first promising safe harbor.

The strongly worded letter was circulated heavily on Twitter and Hacker News, and quickly became a problem for Marc Andreessen, the founding partner of venture firm Andreessen Horowitz, aka A16Z, who maintains a board seat with both Facebook and Caldwell’s company.

Caught in the middle, Andreessen has decided to step down as a board member of Caldwell’s startup Mixed Media Labs (or App.net), as first reported by AllThingsD. A16Z partner Scott Weiss will take Andreessen’s place and assume the Mixed Media Labs board seat for the famous venture capital firm, Caldwell confirmed to VentureBeat.

Caldwell said that he expected as much to happen as a result of publishing the damning diatribe.

“Andreessen Horowitz has, and continues to be, completely supportive of me as an entrepreneur,” Caldwell said. “This move was not unexpected. Scott Weiss is now on my board and I am looking forward to working with him.”

This isn’t the first time the firm’s stake in Mixed Media Labs has made for an awkward situation. Andreessen Horowitz chose to bet on Mixed Media Labs, then focusing on developing photo-sharing app Picplz, instead of re-upping in Instagram. That turned out to be a decision that, while principled, cost the firm millions in proceeds.

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