Off-topic questions with subject matter appropriate to another site fall into three categories: obviously good, obviously crap, and unknown. It's the unknown ones that are difficult. Let me play devil's advocate with the cited question.
Facebook is on-topic at WebApps. This question appears straightforward: can Opengraph be used on Facebook to change Flash to HTML5? For reviewers unfamiliar with Facebook or Opengraph, that looks like something answerable on WebApps by people familiar with the subject matter.
The guideline I personally use is to VTC obvious crap, but vote to migrate questionable questions. The OP came to the SE for help and it seems appropriate to try to get the question in front of the people in the best position to provide it. They are also in the best position to close it if they know that it is a poor question, but at least we tried to help.
If the OP had done any checking on site coverage, they might have originally posted the question on the other site, themselves, so it still would have ended up there. What difference does the route of arrival make? Shouldn't each site be responsible for what it accepts and rejects? Nobody is actively finding crap and migrating it to be obnoxious to another site. And there is the safeguard that five people (or most of five people), need to agree.
Each site has a pool of relevant expertise, which is why we have different sites. Users who don't frequent another site would not be expected to know the nuances of what fits there, so their judgement won't necessarily be reliable on the migration site's behalf. So to some extent, I need to ask whether it is reasonable for one site to expect the users of another site to serve as their filter on questions that are not obvious crap (and what is obvious is not the same for all users).
- Close all but the obvious high-quality questions and advise the OP to repost on their own, and add a pro forma warning to read the site's guidance first. That would be less welcoming, but it would probably serve as a bit of a filter for lazy posters.
- Vote to migrate only if you are a regular user of the migration site or have subject knowledge to know whether the question is answerable there. Otherwise, follow the strategy in the first bullet.
Would that be a preferable strategy?