Your approach is a minimal-effort Pareto principle based hiring. By eschewing any and all candidate information sources other than the market leader professional social network, you get access to a pool of 80% of the candidates with minimal effort. For you, it's great. For your company, it's passable. For the candidates, it's disgusting.
No, at different levels it is the approach everybody takes. The concept of search cost[1] always existed in economy. You can think that an answer to your question is in some obscure web site but if that web site is in the #1000 position in Google, very few people will find it.
Regarding LinkedIn, I think if you want to increase the probability of being hired in US (and other countries where LinkedIn is popular) you must be there. You can argue against it like I argue about needing to have a Skype, or a PayPal account to receive payments: everybody is there, network effects, etc.