Diigo has been around for a long time and is used by many. It has a free tier, which the majority of users use.
Let's have a look at their ahrefs backlink profile to gauge populatiry.
raindrop.io:
Backlinks
190,731
Referring domains
2,748
diigo.com:
Backlinks
50,861,6146
Referring domains
141,090
We can quickly see that diigo is way more popular and linked to on the web. Raindrop is newer and has a better interface.
It depends what you use the bookmark manager for. Is it just for personal use or do you want your content to gain exposure?
My opinion: Diigo for exposure and Raindrop for personal use.