core/homeassistant/components/mastodon/icons.json

{
  "entity": {
    "sensor": {
      "followers": {
        "default": "mdi:account-multiple"
      },
      "following": {
        "default": "mdi:account-multiple"
      },
      "posts": {
        "default": "mdi:message-text"
      }
    }
  }
}