I got them to appear by adding the following additional CSS to my theme:
.heateorSssFacebookSvg {
background: url(‘data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20width%3D%22100%25%22%20height%3D%22100%25%22%20viewBox%3D%22-5%20-5%2042%2042%22%3E%3Cpath%20d%3D%22M17.78%2027.5V17.008h3.522l.527-4.09h-4.05v-2.61c0-1.182.33-1.99%202.023-1.99h2.166V4.66c-.375-.05-1.66-.16-3.155-.16-3.123%200-5.26%201.905-5.26%205.405v3.016h-3.53v4.09h3.53V27.5h4.223z%22%20fill%3D%22%23fff%22%3E%3C%2Fpath%3E%3C%2Fsvg%3E’) no-repeat center center;
}
.heateorSssTwitterSvg {
background: url(‘data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20width%3D%22100%25%22%20height%3D%22100%25%22%20viewBox%3D%22-4%20-4%2039%2039%22%3E%0A%3Cpath%20d%3D%22M28%208.557a9.913%209.913%200%200%201-2.828.775%204.93%204.93%200%200%200%202.166-2.725%209.738%209.738%200%200%201-3.13%201.194%204.92%204.92%200%200%200-3.593-1.55%204.924%204.924%200%200%200-4.794%206.049c-4.09-.21-7.72-2.17-10.15-5.15a4.942%204.942%200%200%200-.665%202.477c0%201.71.87%203.214%202.19%204.1a4.968%204.968%200%200%201-2.23-.616v.06c0%202.39%201.7%204.38%203.952%204.83-.414.115-.85.174-1.297.174-.318%200-.626-.03-.928-.086a4.935%204.935%200%200%200%204.6%203.42%209.893%209.893%200%200%201-6.114%202.107c-.398%200-.79-.023-1.175-.068a13.953%2013.953%200%200%200%207.55%202.213c9.056%200%2014.01-7.507%2014.01-14.013%200-.213-.005-.426-.015-.637.96-.695%201.795-1.56%202.455-2.55z%22%20fill%3D%22%23fff%22%3E%3C%2Fpath%3E%0A%3C%2Fsvg%3E’) no-repeat center center;
}
.heateorSssGoogleplusSvg {
background: url(‘data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20width%3D%22100%25%22%20height%3D%22100%25%22%20viewBox%3D%22-1%20-1%2034%2034%22%3E%3Cpath%20d%3D%22M27%2015h-2v-2h-2v2h-2v2h2v2h2v-2h2m-15-2v2.4h3.97c-.16%201.03-1.2%203.02-3.97%203.02-2.39%200-4.34-1.98-4.34-4.42s1.95-4.42%204.34-4.42c1.36%200%202.27.58%202.79%201.08l1.9-1.83C15.47%209.69%2013.89%209%2012%209c-3.87%200-7%203.13-7%207s3.13%207%207%207c4.04%200%206.72-2.84%206.72-6.84%200-.46-.05-.81-.11-1.16H12z%22%20fill%3D%22%23fff%22%3E%3C%2Fpath%3E%3C%2Fsvg%3E’) no-repeat center center;
}
.heateorSssLinkedinSvg {
background: url(‘data:image/svg+xml;charset=utf8,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20width%3D%22100%25%22%20height%3D%22100%25%22%20viewBox%3D%22-2%20-2%2035%2039%22%3E%3Cpath%20d%3D%22M6.227%2012.61h4.19v13.48h-4.19V12.61zm2.095-6.7a2.43%202.43%200%200%201%200%204.86c-1.344%200-2.428-1.09-2.428-2.43s1.084-2.43%202.428-2.43m4.72%206.7h4.02v1.84h.058c.56-1.058%201.927-2.176%203.965-2.176%204.238%200%205.02%202.792%205.02%206.42v7.395h-4.183v-6.56c0-1.564-.03-3.574-2.178-3.574-2.18%200-2.514%201.7-2.514%203.46v6.668h-4.187V12.61z%22%20fill%3D%22%23fff%22%2F%3E%3C%2Fsvg%3E’) no-repeat center center;
}