What are the sources of earning of musing.io, busy.org, d.tube and other Steem based apps?
Looking at this purely from the point of view of earnings from the Steem blockchain economy, Steem platforms can create revenue streams in a number of ways:
Beneficiaries - Some platforms and Apps for accessing the Steem blockchain take a cut of each post payout using the beneficiaries functionality. This may be 25% for example, although many of the larger platforms have a 0% beneficiaries cut.
Curation - Platforms often use their own Steem Power to upvote users of the platform producing quality posts (which also helps attract new users). The platform will benefit by receiving a share of the curation rewards from each post, typically between 12.5% - 25% of the vote value.
Post payouts - Platforms can fundraise by creating regular blog posts about their services and development. These posts often receive large upvotes (and can be self-upvoted by those platforms with large delegations - although this is a little more controversial).
Delegation - Platforms will sometimes attract a level of delegation from larger users or from the Steemit central team (through MrDelegation for example) due to their beneficial impact on the overall Steem environment and potentially the Steem price. The delegated Steem Power amounts can be very large (1m Steem Power for example). This can increase the three revenue streams above substantially. Many platforms also appeal for SP delegations and pay their delegators a proportion of the revenue generated, resulting in an additional, albeit thinner margin.
Other crypto revenue streams not directly part of the formal STEEM economy include:
Creation and sale of intangible items (such as Steemmonster cards).
SMT economy (a whole set of categories in itself - but not yet launched). These could be linked to a particular platform. Already some Apps such as Actifit seem to be heading down this route.
There are also more traditional revenue possibilities:
Taking a margin on services provided (for example a margin on delegation market, or cryptocurrency exchange) or on goods sold.
Advertising.
There are undoubtedly more. But hopefully that's a good starting point!