The same thing you can do with any subscriber...mail them!
or here's some ideas...
-- promote your primary business opportunity to them.
-- engage with them, ask them for their input, build a deeper relationship.
-- increase the amount of clicks you send in your solo ads.
-- send affiliate offers to them.
-- setup a follow up series that promotes an opt-in form for another email platform. and build your list in even more email services! (we'll even do this for you!)
short answer...do what ever you want!