There's something weird happenning here with that subdomain. I've just tried using that domain to access facebook on my android phone a couple of times, it was redirecting me to https://mobile.facebook.com/?_rdr every time.
Until I've decided to try it in the Chrome Incognito Mode. It worked correctly, without redirecting to the normal facebook's mobile site.
But after coming back from Incognito, it started working as intended on normal browsing mode too.
I noticed something similar when I was travelling in either Hong Kong or Thailand. The network operators in that country offered free access to Facebook and so when accessing mbasic... it would ask you if you wanted to use Facebook for free, and if not then it would forcefully redirect you to the regular Facebook interface - no option to use the lightweight one.
Until I've decided to try it in the Chrome Incognito Mode. It worked correctly, without redirecting to the normal facebook's mobile site.
But after coming back from Incognito, it started working as intended on normal browsing mode too.