Allow me to cut in and mention that even if you do like Social Security in theory, you can't possibly be a Mustachian and support it as it stands. Here is why: it is a PAYGO system.
That means that the so-called Social Security trust fund you've heard of is nothing more than a portal. Each year, money (from taxes) goes in and money goes out in the form of Social Security payments to the program's recipients. If there is an excess of money, it automatically goes to pay down other government debt. (So no, no particular person or group has raided the security trust fund, it happens automatically).
In other words, there is time delay for compound interest to do its thing. The system itself is based around the assumption that at any given time, taxes will cover Social Security payments.
The result is that while current Social Security recipients have had a pretty good deal, the deal the younger generation gets will not be quite the same. This is a matter of math. You cannot have $X in income and pay $1.5X in benefits, not without going into debt. So, taxes must rise, benefits must drop, or the program must be radically restructured so that it is no longer a PAYGO system.
This is not a matter of ideology, these are simple mathematical facts that will hold true unless unforeseen societal changes (f ex, people decide to have more kids again) alter the numbers.
As far as me goes, I am not an ideologue, and I would be alright Social Security if it was set up in a rational manner. For example, my understanding is that the Swedish pension system is set up so that recipients get what current workers can afford to pay--and no more. That is sustainable, Social Security is not. And I don't think anyone, libertarian, Mustachian, or otherwise, should support programs that are so poorly constructed.