by Michael Tarsala

Maybe Facebook should have offered a buy one, get one free offer for shares in its IPO. 

The social media company's market value is now close to being halved from its May 17 market debut, with more traders on Thursday now placing bets whether or not the stock could see $15 toward year's end.

According to Yahoo Finance data, a put at the $15 strike carries the heaviest volume for any Facebook option for December expiration.

The trading activity at $15 – double any other option strike for that month – is one indication that a further slide in coming months is being seen as a potentiality by options market participants.

Facebook (FB) shares are down nearly 5% in afternoon trading Thursday, outpacing declines for the S&P 500 (SPX) and the Technology Select Sector SPDR (XLK). 

Here's a the three concerns that appear to be driving Thursday's move to the downside:

• Katie Mitic, director of platform marketing and Ethan Beard, director of platform partnerships are both leaving the company to pursue other interests. Investors may be asking why they are leaving what was thought to be one of Silicon Valley's premier growth companies so soon.
• The first wave of insider stock-selling restrictions are lifted on August 16. Additional restrictions will go away in October and November. Investors may be concerned that more Facebook shares coming available to be sold could have the effect of pushing the price down further toward year's end.
• Concerns are being expressed about the quality of Facebook's 955 million active users. The company reported in a filing this week that 8.7% of its active accounts -- about 83 million globally – were either duplicate profiles, accounts for people's business or pets, or otherwise undesirable.