Result

シェアボタンのアイコンもCSS製なのでコードは少し冗長化してます

css

.c-share {/*本体*/
  position: relative;
  width: 3.4375em;
  height: 3.4375em;
}

.c-share__input {/*非表示にしておく*/
  display: none;
}
.c-share__input:checked ~ .c-share__toggler .c-share__icon {
  transition: 0s;
  width: 0;
  height: 0;
  color: #639eff;
}
.c-share__input:checked ~ .c-share__toggler .c-share__icon::before {/*クリック（チェック）時に回転エフェクト*/
  transform: rotate(-45deg);
}
.c-share__input:checked ~ .c-share__toggler .c-share__icon::after {
  transform: rotate(45deg);
}
.c-share__input:checked ~ .c-share_options {/*クリック（チェック）されたら各ボタンを格納している要素を展開*/
  width: 11.0625em;
  height: 12.5em;
  border-radius: 0.3125em;
}
.c-share__input:checked ~ .c-share_options::before,
.c-share__input:checked ~ .c-share_options li {/*各展開と同時に要素にアニメーションエフェクト*/
  transition: 0.3s 0.15s;
  opacity: 1;
  transform: translateY(0);
}

.c-share__toggler,
.c-share_options {
  position: absolute;
  right: 0;
  width: inherit;
  height: inherit;
  border-radius: 50%;
  background-color: #f2f2f2;
}

.c-share__toggler {
  cursor: pointer;
  z-index: 1;
  display: flex;
  justify-content: center;
  align-items: center;
}

.c-share__icon {/*シェアボタンのアイコンのスタイル*/
  position: relative;
  left: -0.3125em;
  width: 0.5em;
  height: 0.5em;
  border-radius: 50%;
  color: #029dfc;
  background-color: currentColor;
  box-shadow: 0.625em -0.625em, 0.625em 0.625em;
  transition: 0.1s 0.05s;
}
.c-share__icon::before, .c-share__icon::after {
  content: "";
  position: absolute;
  width: 1em;
  height: 0.125em;
  background-color: currentColor;
  transition: 0.1s;
}
.c-share__icon::before {
  top: 0.1875em;
  left: 0.1875em;
  transform-origin: bottom left;
  transform: rotate(45deg);
}
.c-share__icon::after {
  top: -0.125em;
  left: 0.03125em;
  transform-origin: center;
  transform: rotate(-45deg);
}

.c-share_options {/*各ボタンを格納している要素のスタイル設定*/
  list-style: none;
  margin: 0;
  padding: 1.0625em 1.25em;
  box-sizing: border-box;
  overflow: hidden;
  transition: 0.2s;
  box-shadow: 0 0.125em 0.125em rgba(0, 0, 0, 0.3);
}
.c-share_options::before {
  content: attr(data-title);
  display: block;
  margin-bottom: 1.25em;
  font-weight: 700;
}
.c-share_options li {
  font-size: 0.875em;
  color: #464646;
  cursor: pointer;
}
.c-share_options li:not(:last-child) {
  margin-bottom: 0.75em;
}
.c-share_options::before,
.c-share_options li {
  opacity: 0;
  transform: translateY(0.625em);
  transition: 0s;
}

body {
  background-color: #639eff;
  margin: 0;
  display: grid;
  height: 100vh;
  margin: 0;
  place-items: center center;
}

html

<div class="c-share">
  <input class="c-share__input" type="checkbox" id="checkbox">
  <label class="c-share__toggler" for="checkbox">
            <span class="c-share__icon"></span>
        </label>

  <ul class="c-share_options" data-title="Share">
    <li>Facebook</li>
    <li>Twitter</li>
    <li>Google</li>
    <li>Email</li>
  </ul>
</div>

via

Share button