Stick to your type faces / font families
Although it is fun to play around with different fonts for different moods and different messages make sure that you are using your normal fonts, defined in your visual concept.
Normally this is two or possibly three fonts / type faces. If you mix to much your customer’s and clients do not necessarily recognise this ad, on-line post or flyer/brochure as yours and you might loose business from your regular customers and new clients do not see the continuity and your presence may seem scatty and non reliable.
Of course there are exemptions from the rule but try to at least use most of the message in your normal fonts and then add your special ones as extra effect.