Both companies have refused to comment on the speculation, but talks are thought to be nearing a critical stage.
A decision is expected within the next few weeks, according to reports on Reuters.
StumbleUpon uses recommendations from its 2.3 million users to match websites, videos and photos to a user's interests.
Users can cast their votes using a browser toolbar, giving sites the thumbs up or thumbs down.
"StumbleUpon learns what you like and makes better recommendations," the website claims.
'We are making good progress on 10nm,' claims Intel
Engineer calculates that Chengdu's plan to replace streetlights with artificial moonlight would cost $100bn
Research could also apply to other 'space weather' events involving hot, fast-moving plasma
Dark matter holds the Universe together - and gravitational waves could help identify it