Facebook definitely uses standard tracking pixels (as do nearly all others across the email-sending spectrum). You can also track via click redirection (if someone clicks a link, they can go to your site first, where you record that as evidence of an open, then perform a 301 redirect to another site), but that requires a click, which isn't guaranteed.
At the moment, tracking pixels are the defacto standard. As long as you adhere to good operating principles (pixels should be 1x1, be zero bytes in size, and the HTTP headers should indicate a standard image format and 200 response code), your tracking pixels should operate cleanly.