Twitter stock dropped by more than 10% in early trading Tuesday, falling below $35 a share for the first time since its IPO, after a major stock lockup period expired Monday.

Some 480 million shares held by early investors and Twitter insiders can now hit the market for the first time, significantly inflating the number of shares trading on the market.

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The company has repeatedly tried to defuse investor concerns about the lockup expiration in recent weeks by disclosing that Twitter's cofounders and CEO have no plans to sell any shares. Large shareholders like Benchmark Capital and Chris Sacca have also stated they plan to hold on to their shares.

It's not uncommon for stocks to plummet around the time of lockup expirations as investors grow concerned that insiders will sell off shares en masse and flood the market. Facebook also declined due to concerns over its stock lockup expirations in 2012, but later surged after its biggest expiration didn't lead to a massive wave of insiders selling stock.

Unlike Facebook at that time, however, Twitter is still well above its IPO price of $26 a share, though not by nearly as much as it was earlier this year.

Twitter's stock hit a high of more than $70 in December, the month after it went went public, but it has declined in recent months to its lowest level since the IPO due to concerns about user growth and the stock lockup expiration.

UPDATE Tues. 4:01 PM E.T.: Twitter stock ended the day down nearly 18%, closing at an all-time low of less than $32 a share. More than 120 million shares changed hands throughout the day, a record high trading volume for Twitter since it went public in November.

TWTR data by YCharts