Twitter Inc., which joins the S&P 500 stock index Thursday, is selling $1 billion in convertible bonds.

The move to the S&P will force trillion-dollar index funds to own Twitter stock. Twitter replaces Monsanto Co., which sold to Bayer, on the Index.

Long a struggling cousin to more popular social media sites such as Facebook, Twitter saw two consecutive positive earnings quarters, both the fourth quarter in 2017 and the first in 2018.

Twitter shares (TWTR:NYSE) fell slightly in early morning trading Thursday, down to $39.7 from its 52 week high of $40.16.

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