I have a daughter who's 18, and a son 20, so I know how ubiquitous the texting thing can be. That said, at this age, you have to believe you have raised her with the values that you wanted her to have. Talk to her and keep the lines of communication open, but don't judge her or tell her to knock it off. The reality is that kids constantly text EVERYONE all the time. There may indeed be nothing to it, and she will feel like you are unfairly accusing her and don't trust her judgement.
Although we might not think this is correct behavior, it may be perfectly innocent and fine. Unless you've read the texts and know that they are inappropriate, you should trust your daughter. And, if you've read the texts (oops!) and think they are inappropriate, you still need to let her decide her own rules, with the background of 17 years of your guidance.
She's old enough to know this is right or wrong for her, and only she can judge that for herself now.
It's hard to stand back when you see something you think is going awry, but at this age, you have to let them fly on their own and make their own mistakes.
Good luck, it's not easy!