Twitter Inc. (NYSE: TWTR) keeps turning its doubters into mourners as the stock continues rising and rising. The social media and microblogging site managed a gain of almost 5% on Thursday to close at $73.31. This was the highest close ever, and the new high on an intraday basis is $74.73. One analyst is now saying that enough is enough.

The firm Macquarie downgraded Twitter’s stock to Underperform from an already cautious Neutral rating. The reason: no justification for the recent price surge. Twitter shares are up about 30% over the past week or so, and shares are up about 60% since its initial public offering (IPO).

What was amazing about Twitter’s trading on Thursday was that trading volume hit a whopping 82 million shares. The prior record stands at 117 million shares, but that was the day of the IPO. Maybe Santa Claus sent Twitter shares to all the really good kids or something.

Twitter’s stock valuation has been difficult or impossible for Wall Street analysts to deal with. Anyone with a CFA designation who thought that 50 times revenue after the IPO was too much now gets to consider how to evaluate the stock with a market cap of almost $40 billion and valuation of almost 100 times trailing revenue. Twitter now trades at roughly 62 times expected 2013 revenues and about 35 times expected 2014 revenues. Another negative is that the company is expected to lose money in 2013 and in 2014.

On another note, the December 13 short interest was 23.67 million shares. That short interest may go higher, provided two issues: 1) the cost of borrowing has not risen too high and 2) the short squeeze pain has not wiped out those betting against it.

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