Rdio and MOG, which charge $5 and $10 a month for subscriptions, announced free versions last week in an effort to compete with Spotify. And Rhapsody, whose service costs $10 and $15 a month, has just introduced an array of social features centered on Facebook.

The companies declined to answer questions about Facebook’s media platform. And David Hyman, MOG’s founder and chief executive, said that the development of his company’s free tier far predated Spotify’s entry into the United States.

But Mr. Hyman said that the change was being made to reduce the “friction” a nonsubscriber experiences when following a link posted by a paying user. Instead of hearing the song, the nonsubscriber would reach a page asking to sign up with a credit card — an annoyance for many potential customers.

“In the Internet world, any minuscule piece of friction blows people’s minds,” he said.

MOG provides new users with a “gas tank” of free music — supported by advertising — that increases with that user’s social activity on the site, like sharing playlists or inviting friends. Rdio’s free music will come ad-free.

Neither company would say exactly how much free music would be made available.

“We don’t want to force you to look at or listen to ads that will distract you from enjoying music,” said Carter Adamson, Rdio’s chief operating officer, “and we don’t want you to spam your friends to get more free.”

But even free music requires royalty payments to record companies — typically some fraction of a cent per stream — and some investors and technology executives are concerned that Facebook’s platform may bring in large numbers of users who are willing to listen to some free music but are not being given much incentive to subscribe. That might make success more difficult for services that have less favorable deals with record companies.