I agree with Johand - FNB are trying to attract their user base to use electronic, online means. They know the bandwidth eaters will not use their service as 5GB is not enough for much, so it is actually not a large amount of the user base that is really burning through the current cap as it is. What they do realise is the PR opportunity to use their bulk buying ability to get a better deal for their clients. They are also finding a way to use their data links after hours which are probably fairly dormant, hence the free Youtube and Facebook.
As Johand mentions as well, it is part of a very interesting overall strategy, the VOIP system is actually something that is often overlooked. It is underpinned by Connection Telecom and basically any VOIP to VOIP (any service with an agreement to CT) is free (there are some limits, but you really need to call a LOT to hit these and then the rates are still very good). Then they also do some mobile data via their APN (bulk buying power again) and you get more savings (OK those still need to come down a bit).
Some simple things to think about -
The bank has excess bandwidth after hours,
internet customers are better informed and cheaper than those that come in to the bank for normal banking (and happier),
VOIP means less Telkom lines and simpler PBX design (calls don't need to go via a specific physical Telkom PBX then rerouted across the country and therefore less Telkom lines are needed) as well as reduced call costs and incoming call revenue (interconnect revenue sharing),
mobile banking apps give more functionality and security at a lower cost to FNB (no USSD charges),
paving the way for the next steps (add video to VOIP for example).
in the meantime.... Big ups to FNB, My bank for Life!
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