While Facebook doesn't make the date each user joined the site public, there is a way to find out exactly how long you've been using Facebook. After downloading an easily scrollable HTML file containing the entire contents of your Facebook wall, examine the date stamp on the earliest post; this will indicate when you became active on the social media website.
Log in to Facebook with your email and password.
Click "Account," then "Account Settings." Find the "Download Your Information" section and click "Learn More."
Click the "Download" button. Facebook will now compile a ZIP file of your entire history on the site and send it to the email address associated with the account. This can take a while, especially if there's a lot of data to collect, so don't worry if nothing shows up right away. When the email arrives, you'll be ready to proceed.
Download the ZIP file attached to the email and double-click it to open it. Drag the single folder it contains onto your desktop, then double-click it. Double-click "index.html" to open the downloaded version of your Facebook page in a browser.
Click "Wall" to bring up a page with every single post on your Facebook wall from its beginning. Scroll all the way to the bottom of the page and look at the date on the earliest post. This will indicate when you started using the website.