Let's start with the assumption that anyone who spends more than just a little time on social media is interested in people, and very likely interested in events other people attend. If you make a practice of promoting other business' and other people's events there are two positive results you can count on.
First, if you promote others' events chances are they'll promote yours. Even without asking (but you should) you are likely to find others very willing to cross-promote events.
Second, if you actively promote others' events, it's even more Okay to promote your own. All of this is in the context, of course, that event promotion is a relatively small portion of your social media content.
It also bears mentions what an 'event' constitutes. If a business group is sponsoring a chili cookoff to benefit a charity, that's an event. If someone has an in-store opening of a new line of something (jewelry, clothing, art, vintage wine, or furniture) or is introducing a new chef, that's an event. Only very rarely is a sale an event. As much as people spend advertising money on sales, business people should remember very clearly that social media is not advertising, and not a place to post a continuous stream of ads - doing so will have negative results and likely result in a loss of followers.
I think the best event announcements are those in which excitement and purpose are as much a part of the posting as the actual content. Used in moderation, those can work. If of course, it's overdone and one gushes on a daily basis about one thing after another, that quickly gets old.
If you have something going on in your business that you're really pumped about and just can't wait to tell the world, certainly go for it, but I'd reccomend waiting at least a few days before being so moved again.
One last point on event posting. It's certainly fair and expected to ask others to support your events and promote them - this is done all the time in social media and is seen as an easy way to help other people and businesses. You needn't feel obligated to promote all events nor should you expect that everyone you ask will promote yours. If one of your best friends has just promoted someone else's event and you ask him to do the same for you, she or he may demur, not wanting to overload their own content with promotions.
The bottom line is promoting events on social media is commonplace and a great way for businesses to help each other out. Moderation is the key.
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