Yes, that one is trustworthy...
In addition to the sheer number of users using the extension with an average 5-star rating, note that:
1.) The extension page has the "verified author" logo. That means the extension was really created by diigo.com, not someone just using the name to obtain a false sense of trust.
2.) CNET gave diggo.com its Webware 100 award in 2009, so the site/company is well-ranked and well-trusted.
No harm in that one.