Friends Like These, by Jennifer Lynn Alvarez

Jessica hates Tegan and the only reason she ever agreed to attend Tegan's end-of-the-summer party was because Jake convinced her to go.  Jake's blind to the fact that Tegan (who used to date Jake) wants him back and will do anything to get him.

At the party, Jake and Jessica get separated and, before Jessica can realize what is happening, someone is livestreaming hidden camera footage of Jake and Tegan kissing!  Worse, they have started to take off their clothes!  Utterly humiliated by her boyfriend's very public betrayal, Jessica flees the party vowing to have nothing further to do with Jake or Tegan.  But the next morning, Jake turns up covered in blood with no memory of his infidelity or of anything else that happened.  And Tegan has gone missing.

A rather steamy whodunnit that annoyed me the deeper I got into it. The characters spend a lot of time obfuscating the investigation, which drags everything out and more often than not puts them in a worse position.  There are odd priorities as well, with characters more concerned about statutory rape than homicide.  And finally a large part of the solution to the mystery relies upon information only introduced in the final forty pages of the book, which is frustrating for anyone trying to figure things out along the way.