How can services that use the bitcoin blockchain help secure it?
The Bitcoin network is fairly simple in terms of its core security, so technically there are three things these companies/projects can do to contribute:


Run and/or encourage development of the bitcoind codebase (including code reviews)
Run and/or encourage a higher number of solo miners
Run and/or encourage a higher number of full nodes


Point 3 is especially important and a no-brainer for any project that depends on the blockchain. Running your own full node(s) is a must, so that you don't have to trust a third party node. The tragedy of the commons does not apply to this point.

Each full node adds a layer of security to the network by verifying all transactions and blocks, reducing the chance for foul play, even from "malicious" miners.