Besides using Twitter to:
- build your network and following,
- Twitter can aid in brand/product mentions
- Some reason to believe Twitter links are followed by the search engines, so help with indexation and discovery.[1]
- Some reason to believe Author/Agent/Identity rank of social profiles will start playing a role.[2]
- Finally, there are sites who add Twitterfeeds to the author profiles. Not all these feeds have the "nofollow" property.
The last one is also relevant to the statement: "Wikipedia doesn't matter for link juice". There are many copies of Wikipedia on university domains, where they don't employ nofollow (For study about web crawling or natural language processing). Or people rewriting Wikipedia articles and adding the references without nofollow. A nofollow link can transform into a dofollow link.
>> Don’t use the keyword meta tag.
Exactly, but do use its fine on-page alternative: Microdata keywords: http://schema.org/WebPage
If only for internal usage: Writing down the keywords for a page, keeps you focussed. If you don't mind giving this information to your competitors (there are tools to find out these keywords anyway, if not already obvious), do experiment with microdata keywords.
[1] http://www.seomoz.org/blog/using-twitter-for-increased-index...
[2] http://www.seobythesea.com/2011/11/agent-rank-or-google-plus...