[jkent]

Do you have reason to doubt the validity of this particular survey?

I personally can't see significant problems of this over any survey technique.

There are problems with questionnaires and self-responses, sure, but to say they are worthless is to exclude an entire category of data based on a selective opinion. Horses for courses, if you like.

Many statistical methods are available, some of which are more appropriate than others in particular situations. There is little wrong with convenience sampling if what is of primary importance is convenience, e.g.

(That being said, I'd be careful with taking the results of this poll and applying it to any technology site and therefore the interwebs etc. Inferring from this data that we don't have an ageing population problem would be dangerous).