I've acknowledged other people's anniversaries, but mostly only by phone call, and only a few. I made a card for my brother and his ex-wife, for their first anniversary (just a few months before mine). We threw a party for my grandparent's 50th anniversary, and my parent's 25th. So, mostly milestones for close family. My sister's birthday was on my parent's anniversary, and I don't even remember my parents paying much attention to it, let alone anyone else.
in general, I think anniversaries are more about the couple. I don't think there's anything wrong with a close friend or family member acknowledging them, but it's certainly not something I'd expect.
I agree with this The first few years we were married, family acknowledged our anniversary with a phone call or card in the mail and that was very nice, as newlyweds. After a few years it fell off and we didn't feel slighted in the least. My husband and I celebrate our anniversary as we see fit, and that's fitting to us as it's our special day to commemorate. We don't view it as a day to receive accolades or receive gifts, of all things. That's not why we got married in the first place.
We do acknowledge major anniversaries with friends and family - notably 10, 25 and 50 years, with a card, and if the couple has a party. I've never heard of a party for less than 25 years, and it's never the couple of honor asking for it/throwing it - it's always thrown for them by loved ones (usually the children or best friends organize it), and gifts are never expected. It's just an opportunity for everyone to get together and share their goodwill and affection.