Art/Photography: students can easily post up their digital work on Instagram to show off their work to peers and possible future clients. They can view other's photographs/artwork and get ideas on upcoming assignments. Students can comment on other student work and provide positive feedback to each other.
History: students can create a Facebook page relating to historical events they are learning. Students can role-play by posting status updates in a timeline of events that occurred during a specific era. Students can also have discussions relating to the material. This can allow the teacher to assess students understanding of historical events.
Science: teachers can use Youtube in the classroom to show different models/structures in biology and chemistry. Students will get a better understanding through the visuals and audio from videos on this social network.
English/Government/Journalism: students can set up a Twitter account and follow news sources. This should give students story ideas and allow them to follow up with local stories for their school newspaper. English/government students can express themselves on current controversial topics and post their ideas about important topics.
There a hundreds of other social media sites teachers can use. Its just the matter of how creative you can be with them. I've actually redefined what social media is for me... a platform for a learning network.