div.profile div.username-mobile {
    display: none;
}

@media screen and (max-width: 650px) {
    div.profile {
        width: 100%;
        display: inline-block;
        padding: 5px;
    }
    div.profile div.p_right_top {
        margin-left: 0;
    }

    div.profile div.profile_block {
        padding: 10px;
        margin-bottom: 5px;
    }

    div.profile div.username-desktop {
        display: none;
    }
    div.profile div.username-mobile {
        display: block;
        overflow: hidden;
    }
}