Tag wikis and wiki excerpts have evolved a bit over time. Originally, the excerpt was supposed to be "the elevator pitch" for a tag. However, that guidance has changed and the excerpt is meant to be a quick notice to the Asker about when to use the tag.
The suggested edit reject reason for tag wiki excerpts says:
Simply defining what a [tag] is rarely helps those using it unless the tag's name itself is ambiguous. Excerpts should describe why and when a tag should be used. See the help center for more guidance.
Indeed, the help center has a lot more to say about tag excerpts. (For the sake of brevity, I won't copy it here.)
Since the excerpt is what is displayed in a tooltip when hovering over a tag and when adding tags to a question, it's important that it inform the Asker if they're using it incorrectly.
As an example, the excerpt for facebook, until recently, said:
Facebook is an online social networking service founded by Mark Zuckerberg. If you're developing apps for Facebook, check out StackOverflow.com instead.
It's describing the service, rather than when it should be used on questions here. At least there's some guidance for people who are about to ask a Facebook development question. Presumably the person asking the question already knows what Facebook is; that's not useful.
It might also explain why we're getting questions that clearly belong at Ask Different or Android Enthusiasts instead of here. People see "facebook" and assume that any and all Facebook questions go here.
Compare to the newer version:
For questions about the web interface to Facebook. Questions for smartphone apps or Facebook development belong elsewhere.
This, at least, gives guidance of when to use the tag and a quick mention of when a question might be inappropriate. The description of the service is now in the tag wiki, as is information about where those other questions might actually belong.
Anyway, this is a long-winded way of saying that many of the tag excerpts here lack usage guidance. This is a project to clean them up, starting with our most popular tags.
The answer below is an alphabetical list of our most popular tags that need some help on the tag excerpt. Choose one you feel comfortable fixing, remove the general informational stuff (probably putting it in the tag wiki itself) and adding guidance for when to use the tag as well as when not to use it. (When you've updated a tag excerpt,
<strike> it in the post or mention it in the comments.)
For instance, many web apps (especially Google's) have smartphone equivalents that are probably off-topic here. Also, tags for apps that frequently attract questions ask for support that we can't provide could use guidance that questions here should be general usage questions, but questions of support need to be directed to the owners of the app.
Remember that excerpts can't take markup of any kind, so feel free to link back to an app's main site and/or support site in the wiki.