As a support, much like the jungler, you need to go where you are needed. if your ADC is doing well by himself. he can probably be left by himself. make you you have good ward coverage and he can look after himself. for the most part, if you were wrecking bot lane it's because he's capable. so he can handle himself as long as he has wards.
Janna makes a great map controller. mainly because she's ridiculously fast. you can gank to a degree, but in reality you're probably better off improving the ward coverage up top and buying an oracle and looking at counter warding. buy those pinks, throw them down and start killing wards. if they can't see the map as well as you can, they will be more cautious. if they are more cautious the are easier to crush. don't forget to ward the buffs too. your jungler should be helping you. but in my experience that doesn't happen often. so ward, ward, ward.
Everything else is a judgement call. Janna can't support AP as well as, say taric or Soraka. but she can bring a CC storm in on top of them. you can easily make people blow flash, provided the other lanes are still laning. just dont' forget to go in lane and get some XP now and then.