I think it should count as a surplus in this case when revenue exceeds expenses. After all, the government really has nowhere to put the extra $100 if it takes in $1000 in taxes and only pays out $900 in benefits. They buy treasury bills perhaps, but that's just another way of spending the extra $100, since money you loan the government is spent.
I think, by the way that both democrats and republicans miscount these issues in their own ways. Democrats always want to point to the Social Security trust fund, for example, but that is silly. That money was loaned to the government, and it will require a reduction in spending elsewhere or a raise in taxes elsewhere to pay the money back.
So, while I would count it as surplus when revenue exceed expense (for one year) I would also count it as deficit when expense exceeds income. According to the SSA, that started happening in 2010, "Social Security’s total expenditures have exceeded non-interest income of its combined trust funds since 2010, and the Trustees estimate that Social Security cost will exceed non-interest income throughout the 75-year projection period."
On Social Security statements, they tell you what your benefit will be at your retirement age. They also predict what percentage of benefits will be covered by receipts on that date, based on population dynamics. For my retirement age, that's about 70%. Social Security taxes will only be able to pay a bit more than 2/3 of the benefits promised when I retire.
That's the amount I use for retirement planning, by the way. I don't think Social Security will "go away" but it's inevitable that the amount awarded can't indefinitely exceed the receipts, and I don't think it is sensible to plan for the full amount.