Facebook has 800 million users. Although those with a Facebook profile generally leave their browser open for many hours a day, most are not actually on Facebook to buy items. They are there to connect with friends and socialize. So advertising to the Facebook users requires patience. You should target users based on their interests to create a connection and build a relationship, however you should not try to sell directly to them.
LinkedIn has 100 million users, the majority of which are there for professional reasons. Over one million are high-ranking executives with decision making authority. LinkedIn is best used for business to business marketing.
Reddit receives millions of visitors. If you are not familiar with Reddit, it is a social site that allows users to submit links, then allows other users to vote those submission "up" or "down," so that popular posts are more easily found. The site can be useful to help create viral ad campaigns and cause awareness.
StumbleUpon calls itself a "discovery engine" that finds web content for its users. It "recently passed Facebook as the highest traffic referrer of any website." To catch the StumbleUpon user's attention, Kissmetrics suggests flashy photos and catchy headlines. They tout StumbleUpon's paid platform as possibly the best of all the social media sites.
Twitter is still finding its way in the marketing world. Apparently Twitter users are multi-taskers who are easily distracted. Kissmetrics does warn that you need a fairly large marketing budget to advertise on Twitter. However, if you are building a brand, the return on investment can be significant.
Have you tried advertising on any social media sites? Let us know how it worked for you!