Yup, it’s already on the roadmap!
Remember to rate the plugin 
Okay, just spent some time working on this feature.
Will probably release a new plugin called “Private Messages for Followers Only”. The plugin name is self-explanatory; it’s meant to be used only with the Followers plugin as I don’t recommend using both the friends component and the Followers plugin together.
Very nice r-a-y

Now…. taking things a step further ..
Curious, why don’t you feel Friends and Followers should be used together?
I’m launching a site using both, they both serve different purposes in my opinion… would be ‘ideal’ wink wink, to have it work for both friends and followers…. major pain in the a#$? Because otherwise, yah, you can’t really use both friends and followers with this plugin because obviously you should be able to pm friends….
anyway great stuf!
It’s bad from a usability perspective. Notice that bp.org doesn’t run both friends and followers?
If you want, you can modify the code to support both… although it’s a little harder because there are more variables to consider now… not a friend, but a follower? a friend, but not a follower? Make sense?
I’m not going to add support for both. Sorry!
+1 for the idea of being able to message people who are following you (but not necessarily people YOU are following, if they haven’t followed back)
It could be implemented as a setting, which either the site admin can decide for all users, or users can decide for themselves:
1. Any people on the site can message me.
2. Only people who I am following, who are also following me, can message me.
3. Only people who I am following can message me.
4. Nobody can message me.
Thoughts?
Hi @jossc, sorry for not replying earlier as I didn’t see your post.
Your request is already available in another plugin:
http://buddypress.org/community/groups/buddypress-private-messages-for-followers-only/
I don’t have admin settings allowing the user to choose like you mentioned above though.
Maybe in a future release.
@r-a-y I’m curious about how the code should be modified to support both followers AND friends. I have friends and followers enabled on my site and things seem to be working, but I can understand how this could be confusing. I do like the option of having both though, as they are very different kinds of relationships between users.