Personal anecdote: I will google "<person> Twitter" to get their Twitter feed so I wonder how many of those searches are not necessarily interest in the service rather interest in a specific person using the service.
For Google's "trend" purposes, I'm fairly certain this counts as interest in the service, no? Why would it not?
Since they have separate entries for Twitter and the others, listed as "Social network" vs. just simply "search term" this has to imply some difference in what they include as searches.
So perhaps all permutations of searching for things on Twitter, and others, are lumped into the main search term.