I wrote earlier that information must be transmitted with a "paper-trail" for it to be independently verifiable by every node. The bitcoin block chain is essentially a single, massive "paper-trail" that could potentially be used like a shared database for synchronizing information across nodes.

Using the block chain as a single, shared, decentralized database is an interesting solution but has serious flaws when used as you describe:

1. Transactions are not added immediately (and by added, I mean a consensus has been reached by a majority of the network on the state of the block chain), it can take up to 10 minutes for confirmation to be reached [1] with modern networks and hardware.

2. Each transaction increases the size of the block chain. To independently verify a transaction, each node must have a complete copy of the block chain which is about 21 GB today [2], and growing ~1 GB per month. This rate is expected to increase with time which presents one of the biggest flaws of Bitcoin today. Several strategies exist to reduce the size (pruning of the data, using lightweight clients that only store headers instead of full data) but all of these reduce the ability for an individual node to verify a transaction.

3. Transactions can fail for various reasons: data is too large, transaction fee too low, fork in the blockchain, orphaned blocks, etc. What's even worse is that you won't know (and can't try submitting the transaction again) until confirmation fails ~10 minutes later.

4. Transactions fees for storing arbitrary data in the block chain is dependent upon the real-world price of bitcoin. What's more, once the block reward approaches zero (the reward is cut in half approx. every four years) the rewards of mining will shift entirely to voluntary transaction fees. This will likely cause transaction fees to increase.

5. The block chain is only secure as long as there doesn't exist a single group with a majority of processing power (it is kept in check by competition between miners). If a monopoly develops it can abuse the system very readily.

These various reasons (latency, size, failure, fee increases over time) make the block chain a pretty bad back-end for the implementation of a shared, decentralized database of a real-time P2P app.

The idea isn't completely unsalvageable however, another way of looking at the problem is to realize that the block chain file itself is used as a shared resource between many "threads". Each thread has its own local copy but reconciliation must be performed across all threads to ensure consistent state (which introduces huge amounts of synchronization and contention).

To parallelize the problem, we must reduce the amount of resource contention required. The best way to do that is to use separate databases (eg, separate block chains) for separate, logical clusters of threads. Eg, for a ridesharing app, you can use a separate database (block chain) for each city. This way, only the data that is relevant to each thread must be synchronized.

There's a few problems with this approach (could allow local monopolies to develop more readily) as well but it better emulates natural models for decentralized systems.

It might also be better to use a hybrid approach based on the type of data involved:

1. Use a secure DHT (distributed hash table) to store data that isn't necessarily transactional (such as individual ratings that could be aggregated via an unstructured query).

2. Use a local block chain to synchronize shared resources (such as driver availability).

[1] https://blockchain.info/charts/avg-confirmation-time

[2] https://blockchain.info/charts/blocks-size?timespan=all