The following table summarizes warrant activity for the years ended December 31, 2019 and 2018 (number of shares in thousands):

Warrants issued to Senior Noteholders – On December 22, 2019, in connection with an extension of the Company's senior Notes, the Company issued warrants to purchase an aggregate of 5,500,000 shares of the Company's common stock. The warrants have an exercise price of $1.00 per share and are exercisable commencing June 22, 2020 and expire on December 22, 2021. The Company has determined that the warrants issued in the above transaction did not meet the conditions for classification in stockholders' equity and as such, the Company has recorded them as a liability at fair value. The Company will revalue them at each reporting period, beginning in the first quarter of 2020. See Note 16 for the Company's fair value assumptions.

Nonvoting Common Stock – In connection with the Company's PE Central acquisition, the Company issued nonvoting common shares exercisable at the holders' election. As of December 31, 2019, an aggregate of 3,539,236 shares of nonvoting common stock had been exchanged for an equal number of shares of the Company's common stock upon the holders' request. As of December 31, 2019, 896 shares of nonvoting common stock were outstanding.

At-the-Market Program – In October 2018, the Company has established an "at-the-market" equity distribution program under which it may offer and sell shares of common stock to, or through, sales agents by means of ordinary brokers' transactions on the NASDAQ, in block transactions, or as otherwise agreed to between the Company and its sales agent at prices deemed appropriate. The Company is under no obligation to offer and sell shares of common stock under the program. For the years ended December 31, 2019 and 2018, the Company issued 3,137,392 and 838,213 shares of common stock through its "at-the-market" equity program that resulted in net proceeds of $3,670,413 and $2,056,966 and fees paid to its sales agent of $65,838 and $36,951, respectively. The net proceeds from these issuances were used to prepay a portion of the Company's senior secured notes.