Here's the problem — there are two basic kinds of banner ads out there:
These ads are really just billboards on the (I cringe as I type this) "information superhighway". They act just like outdoor ads, because they only want to make you aware of a new movie, basic message, or consumer brand.
This is pure branding. They don't really need you to do anything besides look, and realize that this is part of an integrated campaign.
2) Direct marketing
These ads are the junk mail of the Internet. They show up uninvited on your homepage, and they want to entice you to act. "Punch the monkey and win a prize!" "Find your best insurance rate!" "Meet hot singles in your area!" Or maybe just finish up your education.
If these ads seem more lurid and crass, it's because they are. Direct marketing is designed to get you to act, and the creative will use any trick in the book to get you to open that envelope — or in this case, to click the ad. They're like a desperate salesman with his foot in the door. Billboards, on the other hand, can take a higher road — whatever the medium.
Both kinds of banner ads are valid marketing strategies, but they can become marketing tragedies if you don't know which kind of banner your ad is supposed to be.
This is something that's particularly difficult in government social marketing, when the message is simple enough to be effectively communicated in a flash, but the campaign is evaluated on how many users click though "for more information".
Billboard or junk mail? It's your call. But you have to make the right one early on, before your objectives come back to haunt you.