My Therapy Couch

Both my boyfriend and my girlfriend suffer from depression and frequently vent to me through text. I am always careful of what I say because you can't always pick up inflection through text, and things can be misconstrued easily. If they ask for advice, I'll give it, but most often it's just them venting and venting their feelings.
If it was in person, I could put a hand on his/her leg or just hold them close so they would know I'm fully there with them and listening. I have a hard time conveying that through text. Other than just saying, "I'm here, I'm listening, etc." do you have any advice on a good way to let them know I'm there and genuinely listening to them? Just saying that seems to annoy both of them, and I don't understand what I'm supposed to say back.