RSUs (even facebook-style) are definitely taxable. With options, you're in control of when to take the tax hit. With facebook-style RSUs, the tax hit comes when the stock gets distributed (taxed as ordinary income) -- usually in the form of withholding some amount of shares.
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Important to remember with RSUs in pre-IPO companies: even though you might get shares distributed at IPO, you're usually subject to a lock up. This is unfortunate because if you receive stock at IPO, you have to pay taxes at them -- so if your company IPOs at $50, then drops to $20 when the lockup expires, you have to pay taxes on the shares you received at $50 value even though you couldn't sell shares at that value.