Is it possible to create a different message for each of my social accounts?

Using the new Tailored Posts Composer, you're able to customise your post per individual social network, but not per individual social account. For example if you're sharing to 1 Facebook Page, 1 Facebook Profile, 1 LinkedIn Page, 1 LinkedIn Profile, 2 Twitter Profiles, and 1 Google+ Page, you'd have four boxes available, as shown below.

You can hover over the social network to the left of the message box to see which social accounts you're creating the message for.