This term would probably not be used in ordinary conversation. It is generally used in a technical sense, as shown by your examples. It means to use an existing system, usually a system of signals or wires, to do something new.
In your example 1, the speaker probably does not mean to say that "they" connected their own network to the system, but rather that they used the existing computers, wires, routers, and Wi-Fi signals when "they" created their own network. Basically this would mean there are two networks which use the same wires; the original network and the new one which is "piggybacking" on the system.
I'm not sure what SOF refers to in example 2.
In example 3, it's more like "spread" than "take advantage of". The accession states have their own economic systems, and the economic upswing can spread across those systems, or "piggyback" on them. Buying Austrian shares causes this to happen.
In example 4, again he is not building a new mobile phone network, but instead he uses the existing signals to spread or create his "intelligent computer network".
For a simpler example, lets say that you have a system of devices which sends oranges from one building to another. Your friend asks you for an apple, and he works in the other building. You decide to piggyback your apple to him using the system which normally sends oranges. So instead of 3 oranges, the other building receives 3 oranges and 1 apple. You used the system in a new way, something it wasn't originally intended to do.