Tiobe is consistent with pretty much any other measuring approach (job boards, github, stack overflow, etc...), so yes. It's a pretty reliable indicator.
Their methodology matters little when their findings are consistent with a lot of other sources that use different methodologies.
All these sites agree on the general brackets that these languages belong to. Until this stops being true, I'll keep accepting TIOBE as a reasonable indicator of language popularity (along with a bunch of other sources).
They give you a rough head count yet you have absolutely no idea what percentage of them are actually good programmers you'd want to hire.
Usually the more popular something is, the more you need to weed out bad candidates as well; its kind of killing the popularity argument.
If you pick a technology based on its popularity or visible activity you're only making it much easier for your competitors to crush you really.
I can only refer to pg's Beating the Averages at this point: http://www.paulgraham.com/avg.html