Recent research shows that in the past year more # trademarks have been filed. Most of them in the US. The hashtag itself does not make a trademark distinctive, so a combination with a descriptive element will not be sufficient to pass the absolute grounds test. For this reason, probably a lot of social media campaigns are not registered as a trademark.
Furthermore, many of these campaigns are short term only, and therefore alone not registered as a trademark. The same goes for campaigns on Twitter. However, if a campaign is used consistently and for a longer period, a registration as trademark is recommended.
(image: The Football club KAA Gent uses the slogan “Come On Blue-White” and on Twitter COBW)