#Social Network Crawler User Agents Users can post URLs on a lot of different platforms nowadays. Most of those platforms will send a request to that URL to generate some preview data from it.
These are a couple of user agents I quickly tested out.
facebookexternalhit/1.1 (+http://www.facebook.com/externalhit_uatext.php)
##Google+
Mozilla/5.0 (Windows NT 6.1; rv:6.0) Gecko/20110814 Firefox/6.0 Google (+https://developers.google.com/+/web/snippet/)
XING-contenttabreceiver/2.0
LinkedInBot/1.0 (compatible; Mozilla/5.0; Jakarta Commons-HttpClient/3.1 +http://www.linkedin.com)
##Delicious
Java/1.7.0_45
Also Twitterbot/1.0
https://developer.twitter.com/en/docs/tweets/optimize-with-cards/guides/getting-started#crawling