Police said they believe the woman was alive as of Wednesday when a call was made through a Skype application on her phone.
The man, who was only identified by his social media name "Taylan," told authorities he first met the woman online a few months ago.
He said they had been in contact Jan. 20, the day before Sierra disappeared. They had planned to meet the following day, the Dogan News Agency reported.
Police said they discovered chat transcripts between Taylan and Sierra through her Facebook and Twitter accounts.
The news agency said the FBI learned the real identity of Taylan and sent the information to Turkish police.
Witnesses have said they saw Sierra after she was reported missing.
She had been expected to return to New York Jan. 22, the news agency reported.