An exceptionally good year for stocks helped drive retail investor bullishness to a three-year high in December, according to a survey from TD Ameritrade released Wednesday.

The company's IMX Index, which began tracking the mood of its 6 million online traders in 2010, also rose for a third- consecutive month in December.

"There's a lot of talk out there about tapering right now, but they are savvy enough to understand if the Fed is tapering, the economic outlook is becoming significantly better," Steve Quirk, senior vice president of TD Ameritrade's Trader Group, said on CNBC's "Halftime Report."

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When looking at 2013 on a whole, for the first time one stock dominated TD clients' monthly trading, and that was Apple.

"If I look at the monthly volume, it was the leader every single month of the year, and we've never seen that before," said Quirk.

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But Quirk hinted that Apple's crown may be in jeopardy with one newcomer recently gaining momentum.

The latest numbers show traders are becoming very active in Twitter.

A closer look at TD's client activity shows investors selling out of Facebook, which hit an all-time high in December, and rotating into Twitter.

Other heavily traded names include momentum stocks such as Tesla and Netflix, while steady-eddy companies like General Electric and Intel were the most widely held.

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"It signals people will buy a portfolio with stocks that are a little stodgy. But what they play with are ones that have more propensity to make money. It's very smart and sophisticated," said Quirk.

TD Ameritrade has more than 6 million funded accounts.

(Read more: Momentum stocks big in October: TD survey)

The company pulls a sample of its users who have completed at least one trade in the last two months and evaluates holdings and positions to calculate individual scores. The median represents its monthly IMX reading.

— By CNBC's Katie Young. Follow her on Twitter @KatieCNBC.