There is a button and it fits right under the profile photo, just above the "send an email button" (and is the same size/shape). However, it needs to be activated to appear.
...Which is weird because, even after de-activating the "send an email" button- which is default-on when you sign up- it doesn't totally disappear, it just fades to a light grey, yet the "send a message" button doesn't show up at all unless you know to look for it with "edit profile".
edit: Also, unlike FB there is no "subscribe" for personal profiles-- you'z either in it to win it
or you ain't. (^c,^)