After some pain, suffering and knots in my brain, I found a clean answer to my multi-account issue (thanks to dependent scope, @Any qualifier and Instance<> class).

I created a qualifier to distinguish different services beans in a polymorphic context. The purpose of this qualifier is only to bind some beans (settings, controller) to a given social service. To be as generic as possible it takes a binding (in opposition to @Nonbinding) parameter which is an enum related to service name. So @RelatedTo(Service.Twitter) qualifies a bean in relation to Twitter. I'm not very happy with this name "RelatedTo" but didn't found something short and eloquent (I avoided @InRelationTo :-) ). Suggestion and commentaries are welcome. (the source is in multi_account feature branch)