I installed Google Drive's "Backup and Sync" app for Mac yesterday, and within a couple of hours I get a mail from Dropbox saying - "We noticed that you haven't added any files recently. Save important PDFs, photos, and work docs to Dropbox to keep your files safe."
Is this just a coincidence or did they react to me installing Google Drive? Thing is I've never got this email from them before, so it made me suspicious!
More likely your installation of Drive was noted by the ad-tech industry. Since Drive is a Google service and Google is in the advertising industry.