There is a lot of speculation it was a BUK system. These have a very restricted operation range, so if so, must have occurred within Ukrainian borders. Most MANPAD systems are un-able to hit something that's in an airway.
Posts have been removed, but it's widely reported these were posted by one of the pro-separatist groups
on their VK
http://oi58.tinypic.com/29gmnah.jpg on their twitter
The guts of the VK post, is claiming that they shot down an an-26, a turbo prop transport, used by Ukrainian military.
This is really going to be a question. The BUK system is speculated mostly because it appears that Russians gave them out like goody bags, even using them in one case to repay a debt to Finland, but Finland doesn't consider them op.
The electronic targeting of the BUK is primitive and almost certainly unlikely to have the newer mode-sierra transponder. As such a blip for the 777, vs an An-26 would be similar. However, the ceiling for the An-26 is about 10,000ft below where the MH flight would have likely been.