The rebels did it, by accident. They thought they had shot down another of Kiev's AN-26 transports (as they had done the previous Monday) and posted a boast to that effect on Facebook. Then they sent troops to the scene to round up any crew who might have bailed out (two bailed out the previous Monday). But they took down their boast when the troops on the ground sent back word that nobody had bailed out and the fields were strewn with dead bodies bearing foreign passports.
Possibly... although the social media you've cited aren't convincing. Lots of people saw the airliner go down or heard the crash and its likely one or more of the rebel groups assumed it was their forces downing a military craft and posted the news.
It could also have been posted by a hacked user account controlled by a third party.
There are a couple of video accounts from people who saw military fighter jets in the vicinity of the airliner just before it went down and the Dutch report doesn't exclude the possibility that Mh17 was strafed with 30mm cannon fire.
Another possibility is that the Ukie military was engaged in a military exercise involving their newly deployed BUK systems and someone pushed the wrong button as happened in 2001...or it could have been a deliberate false flag attack.
Considering how little hard data has been presented by the West, I tend to think the Ukies did it.