The Internet-video site Hulu is adding social-networking functions in hopes of building user loyalty and mining data to attract more advertisers.

Hulu Friends, to be launched Thursday, includes features much like Facebook's, such as the ability for users to create online profiles and share videos with each other. For example, the Hulu "Scorecard" will let users track their activity. Competitors in the online-video market, such as Joost and CBS Corp.'s TV.com, have introduced similar functions.

Hulu...