I'm agreeing with the majority of the thread here, once she does her early heal, you've got about 20 seconds providing something weird didn't happen like jungle invasion gave her your blue. If your support has a skillshot or, better yet, if you do, spam it at the bushes you know she's in. Sorakas are generally not going to move, because they'll assume you just did that to check or to scare them. If there's a ward in there, even better, because you'll be able to see her heal. Once she does that, have your support ready to help you bring down their carry. BUT. You need to remember she also has an under 10sec cooldown silence. This can easily screw up the last skill used to bring someone down, or can prevent your support from healing you in time. She's much slower than the carries early game, as she'll be building towards Philo stone instead of boots, so if it looks like the carry is going to get away, don't hesitate to turn your efforts over to her. Her first rank in ult also has about a 2 minute 40 second cooldown.
Finally, if your health is high enough, wait until her carry goes back. Soraka is easy to beat down on her own, even at her tower if you're careful. I've had many games where I've yelled at my computer "No no don't go back, I can't handle this many creeps on my own". Even with starcall, she's very low damage, but just make sure to do it while her heal is on cooldown