By Rex Crum, MarketWatch

SAN FRANCISCO (MarketWatch) -- A big drop in the stock price of Twitter Inc. stock dominated activity in the tech sector Tuesday as the lock-up period expired for almost 500 million of the social-networking company's shares.

Twitter (TWTR) fell almost 18% to $31.85 a share Tuesday, as many of Twitter's early investors can now choose to sell their holdings. Co-founders Jack Dorsey and Evan Williams, along with Twitter Chief Executive Dick Costolo, have said they did not intend to sell their shares after the lockup period expired.

It was Twitter's lowest closing point since it became a publicly traded company six months ago.

Yelp Inc. (YELP) also had a rough day, as its shares fell more than 13% to $52.13. The shareholders' rights law firm of Johnson & Weaver LLP said it was investigating Yelp and certain company officials regarding "whether certain statements regarding the Company's business practices, prospects, and financial condition were false and misleading when made." Yelp shares have lost almost half their value since early March.

Pandora Media Inc. (P) fell by almost 9% to $22.521 a share. Before the market opened, the Internet radio company reported listener data for April, which included 1.7 billion listener hours, a 30% increase from a year ago, but down slightly from the 1.71 billion hours Pandora reported in March. Pandora also said its share of the total U.S. radio-listening market rose to 9.28% from 7.33% in April 2013.

Declines also came from Netflix Inc. (NFLX) , which fell more than 5%; LinkedIn Corp. (LNKD), down by almost 6% and Facebook Inc. (FB), which ended the day with a loss of more than 4%.

Groupon Inc. (GRPN) shares fell 2.5% to close at $6.72 ahead of the online deal and e-commerce company's quarterly results, which came out after the close of trading.

The Nasdaq Composite Index (RIXF) fell by more than 57 points, or 1.4%, to close at 4,080.

More tech news from MarketWatch:

Twitter tumbles on wave of unlocked shares

Apple's Ahrendts in line for big payday, if she stays the distance

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