I've resurrected an extremely old name for this post as it's all still very close to the bone for me. I had a similar situation a couple of years ago with my uni age daughter, and without going into too much detail it ended up with her being stalked and the police and CPS being involved.
I don't feel comfortable saying too much as it's her story not mine, but it was all horrendous for a while and in some ways still is. And that's the thing that might give your daughter pause for thought. Mine was all very cavalier about it - 'everyone does it these days Mum, it's fine, you don't know anything about it' - but when it went wrong it went badly wrong. As the saying (almost) goes, a stalker is for life, not just for Christmas. The men who are the end users of these services are not always decent, well-adjusted people.
Like lots of girls her age, the internet is an integral part of my daughter's life, which was a factor in how she got drawn into this. Now she can't do anything online with a public profile. All her accounts have maximum privacy settings and she can't use certain platforms. She works in an arts/media field but can't have an online portfolio of her work with contact details because he'll know where she is and where she's been. She can't advertise any freelance services. She can't have a linkedin account. Employers in her field expect to be able to see an online presence and she is hugely disadvantaged by not having that.
She truly thought she was doing something empowering and so much better and cleverer than going out to work for minimum wage in a shop or pub, but just didn't realise the potential implications. I don't think she ever worried about her safety, but I think if she'd been aware that her future freedom could be so compromised she would have thought twice. Once you've given away your privacy and anonymity it's almost impossible to get it back.
Those posters saying ‘my daughter would never do this’, you are supremely unhelpful. OP, you are not a bad mother, and your daughter isn’t bad either. This is a result of a clash between pornsick, misogynist society, the all-consuming influence of the internet in young people’s lives (particularly instagram, in the case of many young women) and libfem ‘sex work is work’, girlpower culture. I’d agree with pp – keep the lines of communication very much open, lots of affection and conversation, listen for subtexts in what she’s saying, and grasp any opportunities you can to boost her self-esteem.
Good luck. Sending strength and solidarity your way. (And to think, we thought the terrible twos were challenging!)