If you believe outside influences are contributing to this situation, first remove said influences. Restrict, or eliminate social media. Does she have a smartphone? Remove it. Facebook account? Suspend it. These artificial feelings of connection ruin adults every day, and they're even worse for young children who need real people to help them make sense of the world. These artificial realities allow them to create private worlds, tailored to whatever they want. Young children do not have the necessary context of real-life experiences to be able to discern between the "healthy" and "unhealthy" messages vying for their attention. Give it all the axe.
Removing these artificial connections will create a vacuum. She will be upset. It will feel like you are ruining the reality she has crafted, and honestly, she will lash out. Be prepared for it. Take it. Own it. Tell her you love her, then move immediately to phase two.
If you are truly committed, you must follow through, and fill this vacuum you created with real human connections. You must be patient, and you must be unwavering. Spend time with her, not in watching TV, or any other "passive" activity. Play games together. Go to a museum. Go to antique stores. Ask her what she likes. Learn her, and find ways to incorporate those likes/dislikes into your time together. You must dig exceptionally deep, and be brutally honest with yourself in finding ways you (yes you Adam) have failed to meet her needs. It's easy to say "someone led our children astray", but if we're really honest, at what point were we as parents no longer their "go-to"? When did they start looking outside the home for fulfillment and validation? Don't beat yourself up here, just be honest in identifying where you missed beats. Speaking from experience, this is incredibly hard. If you are married, your wife will be able to help you here, if you're brave enough to ask. (You know, Pandora's box and all...)
A father has tremendous responsibilities to the well-being and development of his wife and children. This takes an incredible investment of time. Cut back hours at work. Plan things for the two of you, with no other agenda than just to invest in her. Treat it like "courting" your daughter. Most men know what it's like to court a woman. You do surprising things. You take risks. You make sacrifices just because you want her in your life. You give her the absolute best you can. You are patient, and you never, ever give up. You persevere, and eventually, you win that treasured heart.
As men, we tend to attack challenges head-on. If something's broken, we find the problem and fix it. If you frame this in the context of "we have a problem to fix", you will likely fail. That is unless you realize the problem that needs fixing is your relationship with your daughter. The love shared between father and daughter is unique, and more special than anything in the world. It transcends all boundaries. It's like an innocent little romance, unfettered by things we drag around with us in "normal" romances. A father can reach his daughter better than any other person on earth. Capitalize on that. Bring her close to you. Create significant time and space in your life, not to fix her, but to fix your connection with her. Ask her questions. Learn what she thinks about things. As you grow to know her more, all that noise will be drowned out by the calm, loving voice of her father, and she will love you all the more for it.
Sounds hard? It will be, but nothing worth doing is easy. Plan on one to two years of intentional, busting your butt work to bring your daughter close to you. Pace yourself. Create a new "normal". Read books together. Go on bike rides. Adopt a kitten. Involve your wife (if married), but do not neglect that one-on-one time. As you move into the new normal, remember teens need their friends. Bring her friends over to your house, and stay close. Monitor and supervise. Our job is to protect our family from all kinds of things; bad influences are tricky.
I applaud you for standing up for her. Do not be swayed by the world; she was given to you for a reason. Find that reason. Identify your strengths, your weaknesses, what you can do about it, and make every day count.