If I had the reasonable expectation to find somebody qualified willing to help on the street with practically no effort I'd go that route as well. It's just very time consuming to do so, so that's why I usually don't.
That's precisely the point though, it isn't a reasonable expectation. The odds of a postgresql expert just happening to follow them on twitter are poor. The odds of multiple postgresql experts being on the postgresql mailing list is quite high.
Sure, the chances of finding an expert are low, but you'd be surprised how well it works. See, the tweet was posted to HN by someone and the chances of finding a Postgres expert here are significantly higher. More important is that the costs are low, even lower than writing to a ML. So why not try?
