Roggy,
The answer to your question primarily depends on your target market, on your offer, on your industry / niche and on your ideas regarding the right timing based on your ideal customer avatar. Here are some case examples, mentioning just the top 3 networks, based on our recent tests:
CASE A. More clicks and higher conversions with ads in Bing, Facebook and paid placements at authority sites (in this order) for a PC optimization software product developed for personal / home PC use...
CASE B. more clicks and higher conversions with ads in Google, paid placements at authority sites and Google+ (in this order) for a network security software product developed for business / enterprise use by network engineers, IT admins and expert technology staff...
CASE C. More clicks and higher conversions with ads in paid placements at local authority sites, Bing and LinkedIn for specialist B2B services in ICT industry...
CASE D. More clicks and higher conversions with ads in Facebook, paid placements at local authority sites and Google for specialist B2C services in home / office maintenance industry...
Then again, these are our results, our tests, our campaigns. It's best for you to formulate test campaigns, track your results, improve your campaigns, and repeat this process. Cheers!