An ad in the local newspaper, particularly if the base for which you're recruiting is the local market, can announce the virtual job fair's time and online location. Design an eye-catching display ad and run it in the help wanted section of the newspaper. Serious job hunters, as well as those just browsing, tend to look at the Sunday help wanted section. It makes sense, then, that the Sunday prior to your fair would be a good time to advertise. Because people tend to need reminding, you might also run an ad the day before the event.
Radio spots are another possibility.
Keep in mind that saturation is key with radio, and that drive time radio commercials reach the most listeners. It's best to run several spots one or two days prior to the event, rather than spread them out over a longer period of time. More costly, but effective if you're recruiting for a particular industry, are trade publications. Be aware, however, that these kinds of publications often require long lead-times. Although direct mail tends to be expensive, a cleverly designed postcard can make this form of advertising more economical, while sending it to a select target audience can increase its effectiveness.