This may be technically true, but ultimately it's just window dressing. And for FB (and many other companies that play the same game with its more expendable workers), a convenient dodge in terms of both expenses and liabilities.
If you show up at Facebook's building every morning, work in their cafeteria, mop their floors and scrub their toilets -- then for all practical purposes, you're working for Facebook.
I've done contract programming, on site, for a blue collar heavy business. I didn't get invited to their functions either.
Let's please not try to compare the "plight" of contract tech workers to to the situation that bottom-tier contract service workers are in.