It may be that people who follow me on Twitter are disproportionately likely to be pro-McEleny (and maybe even pro-Hepburn), in which case the poll might be underestimating Keith Brown's true support - although bear in mind that the poll was retweeted by 70 people, which hopefully should have given it a wider reach among supporters of all candidates. It's also conceivable that social media polls in general are likely to exclude small 'c' conservative members of the SNP, who again might be more inclined to vote for Keith Brown. However, I still think the poll is a useful exercise, because if the actual result bears no resemblance to the poll result, at least we'll know in future that the balance of opinion on social media is not a reliable guide to the views of the wider membership.
For what it's worth, though, if the actual result is similar to the poll, it seems likely that Julie Hepburn would be declared the winner after second preferences are distributed. I would imagine that many Chris McEleny voters have done the same thing as me and given their second preference to Hepburn. If Keith Brown does top the first preference vote, he's going to need a much more substantial lead to be confident of holding on for the win - and that may be true regardless of whether Hepburn or McEleny is his opponent in the second count.
At the very least we can say that there is considerable uncertainty over who is going to become depute leader, and that on the basis of the very limited information that currently exists, it would be foolish to dismiss the chances of any of the three candidates.