Shares of Fitbit (FIT) - Get Report picked up the pace on Tuesday after a proxy filing revealed a surprise bidding war between Google and a mystery firm that may have pressured Google parent company Alphabet (GOOGL) - Get Report to move forward with its $2.1 billion all-cash offer for the fitness wearable maker. 

In a Securities and Exchange Commission filing submitted by Fitbit after the market close on Monday, the company disclosed that it met with several companies to discuss an acquisition, and that Google and another undisclosed company called "Party A" offered serious bids.

CNBC reported Tuesday that Facebook  (FB) - Get Report was the mystery "Party A" revealed in the filing.

Discussions with "Party A" began on June 11, according to the filing, when the CEO of that company and Fitbit CEO James Park had dinner "and discussed generally the wearables technology landscape."

After a series of discussions with different suitors outlined in the filing, Party "A" on Oct. 12 submitted a non-binding written indication of interest to acquire Fitbit for $7.30 a share. A day later, Fitbit entered into an exclusivity agreement with Google for its $7.35-a-share offer. 

The deal was announced on Nov. 1 when Google parent Alphabet said it would acquire Fitbit for the agreed-upon deal price of $7.35 a share, or about $2.1 billion in cash.

According to company watchers, had there not been a second firm bidding a deal may not have been reached, or Google could have agreed to pay less for the wearable fitness-tracking device maker.

Shares of Fitbit were up 3.83% at $7.04 in trading on Tuesday. 

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