My daughter is 4 next month, started daycare at 9 months, been at the same daycare the whole time and only ever been invited to one birthday party.
I don't think it's an indication of her popularity as it's a small daycare with only 2 rooms. She spent 3 years in the 0-3 room as she was delayed with speech. This is her first year in the big room and no invites yet.
It doesn't really bother me, as I live far away from the daycare and my daughter has been to loads of birthday parties with kids her own age from my mothers' group.