Not all developers have a Facebook account. And the ones that do don't necessarily want their professional activity tied to it. Twitter has no official policy

Not all developers have a Facebook account. And the ones that do don't necessarily want their professional activity tied to it. Twitter has no official policy barring the same person from having multiple accounts (though Github does, for free accounts; thanks to niggler for the correction) --- so, if you want to keep your use of the service from being tied to your other online activity, you can just create a throwaway account. Facebook does; creating "throwaways" potentially puts your use of both Facebook and the other service at risk. More seriously, neither is known for, say, tracking logged-in user activity on other sites (the way Facebook is known to do with "like" buttons).