IMO, the difference in readability has little to do with the difference between OOP and FP. What's important is the expressiveness of the language.

So Ruby can be very easy to read because you can simply keep appending methods to the right.

In Haskell you can do the same, only you'll prepend functions on the left, so you end up reading expressions from right to left:

    (intercalate ", " . map show . Map.elems) myMap

What I find really hard to read, are expressions where the direction of the data flow changes, something that often happens to me in Java:

    Streams.takeWhile(list.stream().map(e -> f(e)), e -> e.isNeeded())