They're still out there but I think the factions meta has slowly died off and changed a lot. Most active ones you'll find are part of a greater network. And they're not the traditional factions you have nostalgia for. They've got custom plugins with classes, items, enchants, ranks, texture packs etc. That's cool and all but I haven't seen an active vanilla factions server in ages.
Just go on YouTube and search "Minecraft Factions" and see what youtubers are playing. They'll always have the server host name in the description. Or go to a server list and sort by Factions only. You'll find tons of results.