Paid traffic sources include search engines like Google and Bing. Retail platforms like Amazon. Social networks like Facebook. Private ad networks like the dating sites. And thousands of independently run, in-house programs, for everything from food, to jobs, travel, loans, etc.
There are also many ad networks to choose from like BuySellAds. They act as a hub where you can choose from hundreds of places to advertise. Some specialize in desktop and web, others focus on mobile and in-game ads.
There are also RTB (real time bidding) networks, that can be used for both advertising and retargeting. The RTB networks make it easy to get your ads into the search engine's content networks, and in the top 10 websites of the organic search results, when you choose to buy remnant ad inventory from them.
You can find curated lists of these traffic sources by searching with phrases like "list of PPC networks". But instead of getting overwhelmed with the details, the whole point of ALOFT is to keep it simple.
After testing Google, Facebook, and dozens of others, it became clear that some ad platforms, or WADS convert better than others. So after crunching the numbers I chose to advertise on Bing's search results and follow it up with retargeting.
If you want more traffic sources, go for it. If you have lots of cash for testing, go big and do PPC on Google search results. If you have a good website with strong landing pages it may be worth it. But keep in mind that we made 1.7 million dollars - take home pay - in 18 months with mostly Bing and retargeting.
Analysis paralysis is a consequence of having too much choice.