Hrm... as I recall, on Friday evening a message was posted to the Steam updates launcher thingy, which brought the Steam votes up to a tie with Zynga, until they spammed all their games again. Then on Saturday yes, Zynga was in the lead, and the polls got pre-emptively closed- I think that the first batch of twitter votes were added at that time as well, however Zynga was still ahead. Then the valve fans went all over getting more votes and it was posted on the steam news to come vote again, and Valve steadily pulled ahead. Zynga had already spammed its fanbases and forum, they'd tapped out the resources of people who would come vote and people became angry at the constant spam and came over to vote against them. Valve's fans were still fresh, many having only seen it on the news that Sunday evening, and Valve kept their lead to the end. Not to mention Zynga voters came and didnt twitter, Valve fans did twitter a LOT.
I see no conspiracy there.