Contributions (T4 boxes 16 and 17) are not deductible on your US return and don't count toward the foreign tax credit. If you are self-employed, you pay into CPP or QPP instead of US self-employment tax.
Benefits (T4A(P), T4A(OAS)): most cross-border professionals treat them as taxable only in Canada under Article XVIII(5) of the tax treaty. The treaty wording doesn't spell this out for US citizens who live in Canada, so the position isn't airtight. US Social Security paid to a US citizen who lives in Canada is exempt from US tax.
Contributions: CPP, QPP and EI
CPP and QPP contributions (T4 boxes 16, 16A, 17 and 17A) are social security taxes. The US has a social security agreement with Canada, and the IRS allows no credit or deduction for social security taxes paid to a country with such an agreement (Publication 514). EI premiums aren't income tax either. None of these reduce your US tax.
Your US return uses the T4's box 14 employment income; the contribution boxes are informational.
Self-employed: no US self-employment tax
Under the US-Canada social security agreement, a self-employed person living in Canada is covered only by the Canadian system. You pay CPP or QPP on your business income in Canada and don't pay US self-employment tax. Attach a certificate of coverage, which you request from the CRA (Quebec residents: Retraite Québec), and write "Exempt, see attached statement" on Schedule 2.
One side effect: income exempt from US self-employment tax this way doesn't count as earned income for the refundable part of the US child tax credit.
Benefits: the treaty position
Article XVIII(5) of the treaty gives the country where a person lives the right to tax social security benefits paid by the other country. It is also one of the provisions the US agreed to honour even for its own citizens (an exception to the treaty's "saving clause").
Most cross-border professionals read this to mean that CPP, QPP and OAS paid to a US citizen living in Canada are taxable only in Canada, and they leave the benefits off US income. An earlier version of the treaty said so expressly for US citizens; the wording was changed in 1997, and the current text speaks of benefits paid to a resident of the other country. The IRS has not issued guidance on this exact case. That is why the position is common but not airtight.
If you include the benefits instead, the Canadian tax on them counts toward your foreign tax credit, and many people end up owing little or no US tax on them either way.
Form 8833. Treaty positions on social security and pensions are exempt from the Form 8833 disclosure requirement (Treasury regulation 301.6114-1(c)(1)(iv)). Some filers attach it anyway, to show the position openly.
US Social Security while living in Canada
If you are a US citizen living in Canada and receive US Social Security, IRS Publication 915 says the benefits are exempt from US tax. Canada taxes them instead, with 15% of the benefit exempt from Canadian tax.
FBAR and Form 8938
CPP, QPP and OAS are government benefits, not accounts. They don't go on the FBAR, and the Form 8938 instructions exclude foreign social security from reporting.