2 years is a very loooong time! Several things that are working in your favour:
1. Facebook makes revenue from advertising. People are being increasingly turned off by advertising. As people close accounts, the management gets worried and pushes advertising even more. Advertisers buy eyeballs/clicks/impressions - whatever - it's all about the numbers. Less users, means less value, hence less revenue to FB.
2. The cool kids don't use what they parents and grandparents use. So as the "oldies" join, the youngsters drop off. But the oldies don't like advertising either ...
3. FB does NOT pay dividends. So why would you buy shares other than to sell at a profit? Which sooner or later ends up being a re-run of the Tulip Bulb craze. As soon as there are no buyers to be found at a particular price point, the prices collapse and increasing numbers of people try to offload which then pushes prices further down.
Caveat: I'm an engineer not a stock broker nor an accountant, the following is my back of the envelope calcs.
4. If FB responds to market pressure and decides to pay a dividend, then based on current revenues of $3b and 2.74b shares, you could expect a dividend around 50c which, looking WPP Holdings 2014 financials for comparison (eps 85, div 38, sp 1400), would have a FB share price approx $20 - compare this to currently at $105 ($38 at IPO).
Based on the above, the users and investors will bring about the demise you need to safeguard your bet.
BTW: Did you see the 1983 movie "Trading Places" with Eddie Murphy and Dan Aykroyd?