I've had experience of recruitment agencies in the past and recruiters getting in touch with me out of the blue on Linkedin although I find that they can be a bit of a nuisance in terms of taking your contact details and never getting back in touch or sending you job ads that are not relevant or not exactly what you're looking for. If they're a good recruiter they will take time to understand your skillset and what jobs you're interested in but a lot of the time I find they're more focussed in filling roles to get their commission at the end of the month!