Our lot have been selling online since 1999. We track all sales so we know what is cost effective and what isn't, ROI is measured. The only online spend that works well for us (specialised sports products) is Google Shopping, followed by very carefully controlled Adwords campaigns that don't go anywhere near associate sites. Facebook is a complete waste of money - we've tried it up, down and sideways, it is useless.
If you employ an agency to run campaigns, be very, very careful as most do not have a bulls notion of how the interweb works, they just want to pick up their commission cheques and client fees in exchange for some impressive graphics. It is a minefield and great way of making someone else rich.