How do Functional Reactive Programming and the Actor model relate to each other?
Neither actors nor FRP are about streaming. Actors don't even support external configuration of an output stream. 

FRP is strongly characterized by its modeling signals and events on a linear timeline, which enables FRP behaviors to compose in a deterministic manner. Actors are strongly characterized by processing messages in non-deterministic order, and have scarcely any compositional properties (i.e. you can't treat an arrangement of two actors as a larger actor).

If you're seeking similarities, both actors and FRP have a close relationship to lambda calculus. Both can model systems responsive to human input. Both support modeling of internal (local) state. 

FRP supports local state through integrals or accumulators (fold over time), while actors model supports state by allowing each actor to specify its behavior for the next message in response to the current one. This pervasive support for local state makes both FRP and Actors inadequate for live programming (or runtime upgrade of program code); it becomes too easy to lose important state.

Regarding application domains: 

Actors model is well suited for open systems, where we might wish to install or maintain actors at runtime. Actors model is also weakly suited to distributed systems, as the non-deterministic ordering of messages can make a conforming implementation easier. (The reason actors are not more strongly suited to distributed systems is that ensuring a message arrives 'once and only once' is quite difficult in the face of disruption, and actors also tend to require distributed GC, which is a pain.)

FRP is well suited to closed systems that operate over time - e.g. robotic controllers, music programming, computational toys. The determinism and compositional features make FRP more convenient to work with than actors, at least in those cases where FRP can directly model a solution. Integrating FRP with effects (elegantly, without hacking the model with impurity) has proven difficult. There has been recent work on effectful FRP via 'wormholes' - effectively, unique or linear typed access to resources.

There are other models that lie somewhere between FRP and Actors.

Flow Based Programming (FBP), developed by John Paul Morrison, really does support streaming of messages.

Time Warp protocols (or the more recent work on Lightweight Time Warp (LTW)) places actors-like messages on a logical timeline to provide a more controlled and compositional notion of message passing. Time warp is often used for large parallel and distributed systems, e.g. scientific computing. The original time warp was unsuited for interactive simulatons (responsiveness to human input), and LTW is only marginally suited.

I am developing Reactive Demand Programming (RDP) which enables responsive, compositional, FRP-like manipulation and processing of signals in open and distributed systems, and eliminates local state. RDP is achieved by constraining side-effects to commutative, idempotent influence on resource state by signals over time. RDP requires rethinking of resource and state models.