Well, there's no other visible revenue source currently, but (a) we don't know what the ad/user data revenue looks like down to the finer details¹, and (b) theoretically they could have planned to turn it into a subscription service (pretty unlikely, but who knows…)

¹ some information could be gleaned from public filings, but the more interesting question is the relationship between amount of data extracted and revenue gained; any public filings only show the status quo.